Koi / Nishikigoi case method: 50 questions

One morning in early May, in the garden pond (water 22°C), one koi has died on each of two consecutive days. The remaining 10 are motionless on the bottom, moving their gill covers rapidly. On the dead fish the gills have turned white and partly dissolved, and the eyes are sunken. Last month a new koi bought at a sale was put straight into the pond without quarantine. You have bagged the dead fish, thinking of consulting an acquaintance's koi farm.

Which is the most appropriate first action in this situation?

1. Drain the whole pond, refill with fresh tap water, and release the remaining fish into a nearby river to save them
2. Take the dead fish to the koi farm to have the cause investigated and leave the pond alone until results come back
3. Do not drain pond water outside or move the fish; report the same day to the prefectural fisheries division or the Livestock Hygiene Service Center
4. Add double the labeled dose of fish medication to the pond and move only the healthy fish to another pond to watch

AnswerC. Do not drain pond water outside or move the fish; report the same day to the prefectural fisheries division or the Livestock Hygiene Service Center

ExplanationA run of sudden deaths in spring or fall at 18–28°C with gill rot and sunken eyes strongly suggests koi herpesvirus disease (KHV). KHV is a notifiable disease under the Act on Ensuring Sustainable Aquaculture Production; reporting to the prefectural government is mandatory on detection, and release or movement of fish is prohibited. The correct response is to keep the water in and the fish in place, report the same day and wait for instructions. Releasing fish into a river spreads the virus and is illegal release, the worst choice. Taking fish to a koi farm risks spreading infection to other ponds. Overdosing medication has no effect on KHV and only weakens the fish.

IssuesThe biggest risk factor was putting a new fish into the pond without quarantine. The owner did not know that KHV is notifiable, with a duty to report and a ban on release, and nearly made the well-meant decision to release fish or take them to another farm. No water test or cause analysis was done when fish began dying.

ImprovementQuarantine new fish in a separate container for 3–4 weeks and add them to the pond only after confirming no abnormalities. Post the contact details of the prefectural fisheries division and Livestock Hygiene Service Center beside the pond. Share the signs of suspected KHV (sudden deaths in spring/fall, gill rot) with the whole family, and set rules: seal dead fish in bags and never drain pond water outside.

At 5 a.m. on an August morning, you go out to the pond and all 12 koi are gathered at the surface gasping (piping). The waterfall pump had been switched off overnight. The previous day was a 35°C heat wave with water temperature above 30°C, and last night you fed an extra-large portion of leftover pellets after dinner. The fish can still swim but are sluggish.

Which is the most appropriate first action?

1. Immediately run the pump and aeration at full power and change one third of the water with new water adjusted to the same temperature
2. Assume it is lack of oxygen and pour in large amounts of cold tap water with a hose to drop the temperature at once
3. Piping is begging behavior, so give a little food to calm them down
4. Once the sun is up the plants will produce oxygen, so finish breakfast and come back to check later

AnswerA. Immediately run the pump and aeration at full power and change one third of the water with new water adjusted to the same temperature

ExplanationPiping is a sign of low dissolved oxygen. Warmer water holds less oxygen, and at night plants and algae also consume it, so oxygen is lowest in the early morning. This is a classic case compounded by switching off the pump overnight and the extra organic load from overfeeding. Immediately maximize aeration and water flow, and do a partial change with new water at the same temperature to restore oxygen and water quality. Dumping in cold tap water causes shock from the sudden temperature change (the guide is within 2°C per day) and chlorine damages the mucus layer and gills. Feeding increases oxygen consumption and fouls the water. Left alone, suffocation deaths begin during the morning, so waiting is dangerous. If there is no improvement, consult a fish vet the same day.

IssuesThe pump was switched off on a scorching night, cutting off the oxygen supply. The owner did not understand that dissolved oxygen falls at high temperature and increased oxygen demand and water load further with heavy evening feeding. There was also the knowledge gap of mistaking piping for hunger.

ImprovementRun the pump and aeration 24 hours a day in summer and keep a battery air pump on hand for outages or breakdowns. On days above 30°C, shade the pond and reduce or stop feeding. Share piping and rapid gill movement as danger signs with the family, and make a habit of measuring early-morning oxygen with a dissolved-oxygen meter or test kit.

In November, a 3-year-old koi kept in an indoor 90 cm tank has had a swollen belly for several days, and this morning its scales are standing out like a pinecone. It is not eating and sits motionless on the bottom. The two tankmates are fine. The filter has not been cleaned for six months. A water test shows ammonia present.

Which is the most appropriate response?

1. Press the scales back down with your fingers, then crush human antibiotics and mix them into the food
2. To avoid infecting the tankmates, release the affected fish into the outdoor pond to isolate it
3. Raise the water temperature to 30°C at once to boost immunity and feed more to build strength
4. Isolate in a separate container with a 0.5% salt bath and no food, and change the main tank's water. See a fish vet the same day or next day

AnswerD. Isolate in a separate container with a 0.5% salt bath and no food, and change the main tank's water. See a fish vet the same day or next day

ExplanationRaised scales with a swollen belly are classic dropsy (pinecone disease), where fluid accumulates in the body due to poor water quality and bacterial infection. As the ammonia reading shows, the cause lies in water quality, so isolate the fish, give a 0.5% salt bath to reduce osmotic stress, withhold food to spare digestion, and change the main tank's water and clean the filter. Severe cases rarely recover, so consult a fish vet the same day or the next. Pressing the scales damages mucus and skin; self-dosing human drugs is dangerous with unknown dosages. Releasing into the outdoor pond spreads pathogens and may count as illegal release. Sudden heating causes temperature shock and feeding fouls the water further.

IssuesThe filter went uncleaned for six months and water was not tested until ammonia was found. The early sign of a swollen belly was missed for several days until the scales stood up. No isolation container or salt was ready, and dangerous options such as human drugs or release were considered.

ImprovementFix monthly water testing (ammonia, nitrite, pH) and regular filter cleaning on the calendar. Keep an isolation tub, air pump and pre-measured salt on hand, and post the recipe for a 0.5% salt bath (50 g of salt per 10 L of water). Check appetite and belly size at every morning feeding.

In late March, after a string of days when the outdoor pond was 8°C in the morning and colder at night, several 2-year-old koi have started showing white spots on their bodies and fins. The fish rub against the pond walls and bottom and swim with fins clamped. Last week you added ornamental aquatic plants to the pond. The white spots are increasing day by day.

Which is the most appropriate response?

1. The white spots are winter mold and will disappear when it warms up. Do nothing until spring
2. Avoid sudden temperature changes, isolate the affected fish in a container with a 0.5% salt bath, and see a vet within a few days if spots increase
3. Rub the white spots off with a soft cloth, apply antiseptic to the wounds and return the fish to the pond
4. Dump several kilograms of salt into the whole pond to make strong brine and kill the parasites all at once

AnswerB. Avoid sudden temperature changes, isolate the affected fish in a container with a 0.5% salt bath, and see a vet within a few days if spots increase

ExplanationWhite spots and body rubbing are classic white spot disease (a ciliate parasite). A sudden drop in temperature or new plants bringing in the parasite can trigger it. Isolate the affected fish, avoid sudden temperature changes, reduce stress with a 0.5% salt bath, and consult a fish vet within a few days if spots increase or appetite is lost. Left untreated, the parasite reaches the gills and the fish dies of respiratory failure. Rubbing with a cloth strips the mucus layer and invites secondary bacterial infection. Dumping large amounts of salt into the whole pond cannot be controlled, harms the filter bacteria and other fish, and makes the whole pond worse.

IssuesNew plants were put straight into the pond without quarantine, very likely introducing the parasite. The owner did not know that early-spring temperature swings trigger white spot disease and did not recognize body rubbing as an early sign. There was no isolation container, and dangerous measures such as medicating the whole pond or physical removal were considered.

ImprovementQuarantine plants and new fish in a separate container for 2–4 weeks and observe before adding to the pond. In spring and fall check the thermometer daily and feed lightly during periods of large swings. Keep an isolation container, air pump and pre-measured salt ready, and set a rule to consult a fish vet early once white spots appear on 2–3 or more fish.

In October, in an outdoor pond at 17°C, a 5-year-old Taisho Sanke has a red swollen area on its flank; scales have come off, a hole has opened in the skin and white muscle is visible. The fish is swimming but eating less. Last week you may have scraped its body when netting it for a photo. The pond has accumulated fallen leaves and sludge on the bottom.

Which is the most appropriate response?

1. Isolate in a container with a 0.5% salt bath, keep the water clean and consult a fish vet the same day. Change the pond water and clean the bottom
2. Apply a thick layer of human ointment to the wound and leave the fish in the air for about 10 minutes to let it dry
3. The hole will heal on its own; switch to high-protein food and watch it in the pond
4. While removing the leaves, drain the pond completely and refill directly with tap water for a full reset

AnswerA. Isolate in a container with a 0.5% salt bath, keep the water clean and consult a fish vet the same day. Change the pond water and clean the bottom

ExplanationHole-in-the-side disease, common in spring and fall at 15–20°C, is a bacterial infection triggered by wounds or poor water quality. Exposed muscle means it is advanced, so isolate the fish in clean water with a 0.5% salt bath and consult a fish vet the same day for proper treatment. At the same time change the pond water and clean the bottom, since leaves and sludge are breeding bacteria. Leaving a fish in the air dries the mucus and gills and is fatal, and human ointment washes off and does nothing. Left alone, the hole spreads and the fish dies. A full water change loses the bacteria, and chlorinated tap water damages the mucus and worsens the condition.

IssuesNetting caused an abrasion, and the pond was left full of leaves and sludge where bacteria multiply easily. The owner did not know this was the temperature band when hole-in-the-side disease is common, and was slow to act on the early sign of reduced appetite.

ImprovementCatch fish underwater using a plastic bag or rubber net so the body is not scraped. Stretch a leaf net in fall and siphon bottom sludge monthly. During the 15–20°C period in spring and fall, check daily for red swelling or scale abnormalities and keep an isolation container and salt ready. Look up a fish vet's contact details in advance.

In September, two weeks after adding a koi bought at a sale to the pond, several other koi keep rubbing their bodies. On close inspection, thread-like worms about 1 cm long are embedded in the skin, with red bleeding at the base. Another fish has flat disc-shaped creatures clinging to it and moving.

Which is the most appropriate response?

1. The worms will drop off by themselves in time; increase food to keep the fish strong and leave them
2. Scrape the worms off yourself with the tip of a box cutter and dab household mouthwash on the wounds
3. Dilute garden insecticide and add it to the pond to wipe out all the parasites at once
4. Consult a fish vet; have anchor worms removed by a professional and have a deworming plan drawn up for the whole pond

AnswerD. Consult a fish vet; have anchor worms removed by a professional and have a deworming plan drawn up for the whole pond

ExplanationThe thread-like worms are anchor worms and the disc-shaped ones are fish lice; both are mainly introduced by new fish. An anchor worm's head is embedded deep in the skin, and if an amateur pulls it out the head remains and the wound festers. Leave tweezer removal to a fish vet or an experienced professional, and discuss a deworming plan that also covers larvae remaining in the pond. Left alone, the hemorrhagic spots become infected, the parasite load grows and the fish weaken. Garden insecticide is highly toxic to fish and can wipe out the whole pond; never use it. A box cutter or mouthwash damages the fish's mucus and skin.

IssuesA new fish was put in the pond without quarantine, spreading parasites to every fish. The owner did not know the types of parasite or correct removal methods and considered lethal measures such as human antiseptics or insecticide. Body rubbing was missed as an early sign.

ImprovementStrictly quarantine new fish for 3–4 weeks and watch for parasites during that time. Clean leaves and mud from the pond to reduce parasite habitat. Look up a fish vet's contact details and consult rather than self-treat when parasites are found. Make a family rule never to use insecticides or pesticides near the pond.

In July, a 1-year-old koi in a tank is working its mouth near the surface, its gill covers moving rapidly, and the right gill cover stays shut and does not move. Looking inside, the gill appears dark. The tank is 60 cm with five young fish and one small hang-on filter. A nitrite test gives a high reading.

Which is the most appropriate response?

1. Pry the gill cover open with your fingers, wash inside, rinse directly under tap water and return the fish to the tank
2. Add aeration, change one third of the water with new water at the same temperature to lower nitrite, then consult a fish vet the same day
3. The fast breathing is due to heat, so put ice packs directly in the tank to cool it quickly
4. Changing the tank would startle it, so do nothing and observe for a few days before deciding

AnswerB. Add aeration, change one third of the water with new water at the same temperature to lower nitrite, then consult a fish vet the same day

ExplanationOne gill cover not moving, rapid breathing and darkened gills are signs of gill disease (gill rot), suggesting gill damage from nitrite or ammonia buildup plus bacterial infection. Five fish in a 60 cm tank with one small filter is clearly overcrowded and under-filtered. First add aeration to secure oxygen, do a partial change with new water at the same temperature to lower nitrite, and consult a fish vet the same day. Gills are extremely delicate; prying them open or rinsing with tap water destroys the tissue and is fatal. Rapid cooling with ice packs causes temperature shock. Gill damage leads to suffocation within days, so waiting is dangerous.

IssuesThe root cause was overcrowding, five koi in a 60 cm tank, and under-filtration with one small filter. Water was not tested until nitrite was high, and the owner did not grasp that koi grow fast and need a large water volume.

ImprovementEven young koi need a tank of 90 cm or more or a pond, and must be moved as they grow. Upgrade filtration to at least one third of the water volume and keep to weekly one-third water changes. Test nitrite and ammonia weekly, and check at daily feeding for rapid or one-sided gill-cover movement.

In mid-January the outdoor pond is at 6°C. The koi are gathered in the deep area on the bottom, barely moving, and ignore food thrown in. Their bodies look normal and their gills move slowly. A worried family member says they will starve and proposes raising the temperature with a heater and feeding them.

Which is the most appropriate response?

1. Raise the temperature to 20°C overnight with a heater to restore appetite and feed them well
2. Not eating means illness, so move them all to an indoor tank and add fish medication
3. Below 10°C it is normal for koi to fast through winter. Do not feed; if the pond freezes, keep a hole in the ice and watch quietly
4. Worried about leftovers, throw in small amounts of sinking food every day and wait for them to eat

AnswerC. Below 10°C it is normal for koi to fast through winter. Do not feed; if the pond freezes, keep a hole in the ice and watch quietly

ExplanationKoi are cold-blooded; below 10°C their digestion slows and it is normal physiology to sit almost motionless in the deep area, fasting through winter. With no body abnormalities and slow gill movement, this is not illness; just watch quietly. In freezing regions keep a deep area and open a hole in the ice to maintain gas exchange. Raising the temperature to 20°C overnight causes shock from the sudden change (guide: within 2°C per day), and immunity cannot keep up, inviting disease. Moving large fish indoors is highly stressful, and medicating healthy fish is pointless and harmful. Food given at low temperature cannot be digested and causes enteritis, and leftovers foul the water.

IssuesThe family did not know that koi fast through winter at low temperature and mistook not moving or eating for abnormality. The rule for stopping winter feeding (below 10°C) had not been shared within the household. There was a lack of awareness that rapid heating is dangerous.

ImprovementInstall a thermometer in the pond and post the rules for the family: reduce feeding to once a day below 15°C and stop below 10°C. Keep a deep area (1 m or more) in winter and keep part of the ice open in freezing regions. Do not catch or clean during dormancy, and let the spring warm-up proceed gradually at no more than 2°C per day.

In June, 15 koi are kept in a pond at 24°C. At morning feeding, one Showa Sanshoku has left the group and sunk to the bottom in a corner, fins clamped and motionless. There are no obvious wounds or white spots, but its color looks slightly duller than yesterday. The other fish are eating well. Water was tested last month with no problems.

Which is the most appropriate response?

1. Retest the water, isolate in a separate container with a 0.5% salt bath and observe. If no improvement, consult a fish vet the same day
2. Chase it with a net to the center to rejoin the group and scatter food in front of it to make it eat
3. The others are fine so it is not sick. Leave it for a few days to recover naturally
4. Its color has dulled, so increase color-enhancing food and dose the whole pond preventively with fish medication

AnswerA. Retest the water, isolate in a separate container with a 0.5% salt bath and observe. If no improvement, consult a fish vet the same day

ExplanationLeaving the group and sitting in a corner with fins clamped is an early sign of poor health, and dull color indicates mucus abnormality or stress. Even if the other fish are fine, this individual is likely at the start of a disease or internal problem. Water quality changes daily even if last month's test was normal, so retest, isolate the fish in a separate container, reduce stress with a 0.5% salt bath and observe; if there is no improvement, consult a fish vet the same day. Chasing a weakened fish with a net is highly stressful and causes injury. Early signs left alone often become too late within days. Preventive dosing of the whole pond with the cause unknown destroys filter bacteria and burdens the other fish.

IssuesThe assumption that all is well if the other fish are fine led to downplaying early signs in an individual. Water was tested only monthly, with no habit of retesting when something was off. It is unclear whether an isolation container was ready, and no first-response procedure had been set.

ImprovementMake a habit of checking every fish's position, fin posture and appetite at morning feeding. Write a procedure on paper beside the pond: on finding an abnormal fish, retest water the same day and observe in an isolation container with a 0.5% salt bath. Set a recovery criterion (fins open and eating within 24 hours) and see a vet if it is not met.

In April, the tail fins of several 2-year-old koi in an outdoor pond have turned cloudy white and the edges are rotting into a ragged shape. One has lost about half its tail fin. It is spawning season, and you have repeatedly seen males chasing females and hitting the pond edge. The pond water is slightly cloudy and has not been changed for a month.

Which is the most appropriate response?

1. Trim the rotted fins neatly with scissors, apply antiseptic to the cut edges and return the fish to the pond
2. It is a natural part of spawning season; leave it and think about it after spawning ends
3. Add a small amount of household bleach to the pond to sterilize the water and clear the cloudiness
4. Improve water quality with a water change, isolate the worst-affected fish in a 0.5% salt bath, and see a fish vet within a few days if rotting progresses

AnswerD. Improve water quality with a water change, isolate the worst-affected fish in a 0.5% salt bath, and see a fish vet within a few days if rotting progresses

ExplanationCloudy, rotting fins are fin rot (bacterial infection), brought on by injuries from spawning-season chasing combined with poor water quality after a month without a water change. First improve water quality with a water change, then isolate the worst-affected fish in a 0.5% salt bath to control secondary infection. If rotting reaches the base of the fin the infection spreads to the body, so consult a fish vet within a few days if it progresses. Cutting with scissors widens the wound and antiseptic damages the mucus. Spawning injuries themselves are natural, but bacterial infection worsens if ignored. Bleach is highly toxic to fish and filter bacteria; never use it.

IssuesDuring spawning season, when males chase females and injuries increase, water changes were neglected for a month and water quality deteriorated. No thought was given to spawning-season crowding or sharp pond edges as causes of injury, and fin abnormalities tended to be dismissed as natural.

ImprovementBefore spring spawning, change the water, clean the pond and pad sharp edges and pipes. When chasing is intense, temporarily separate females into another container to reduce injuries. Fix weekly one-third water changes and check every morning for cloudy or ragged fins as early signs. Keep an isolation container and salt on hand.

In September, in a pond at 25°C, an 8-year-old Kohaku takes food into its mouth but spits it right out. Its belly has been slightly swollen for several days and its feces are white, thin and stringy, floating at the surface. Through the summer you kept using high-protein color-enhancing pellets from a bag opened three months ago. The bag was kept in an outdoor shed and had become damp.

Which is the most appropriate response?

1. To restore appetite, offer its favorite bread or boiled rice so it eats at least something
2. Stop using the old food, fast for a few days, isolate in a 0.5% salt bath. If white feces and swelling persist, see a fish vet within a few days
3. Raise the temperature to 30°C at once to aid digestion and keep feeding the same food, softened
4. Spitting out means it is bored with the food. Switch types and double the amount

AnswerB. Stop using the old food, fast for a few days, isolate in a 0.5% salt bath. If white feces and swelling persist, see a fish vet within a few days

ExplanationSpitting out food, white stringy feces and a swollen belly are signs of digestive trouble (enteritis or internal disease). Food gone stale from humidity develops mold toxins that cause liver damage and enteritis, so first stop using the old food. Fast for a few days to rest the gut, isolate in a 0.5% salt bath to reduce stress, and consult a fish vet within a few days if symptoms continue. Bread and rice cause indigestion and foul the water; never feed them to koi. Sudden heating causes temperature shock, and continuing the old food keeps the toxins coming. Increasing the amount only burdens the gut more.

IssuesDamp high-protein food from a bag opened three months earlier was used continuously, causing internal damage from mold toxins and oxidation. The food was stored in a hot, humid outdoor shed. The early signs of a swollen belly and white feces were ignored for several days.

ImprovementBuy small bags that can be used up within 1–2 months of opening, store sealed in a cool dark place and discard damp food. Use high-protein summer color food only while water is warm, switching to easily digested food in spring and fall. Check feces color and shape daily, and fast and isolate immediately on seeing white stringy feces or spitting.

In May, pond at 20°C. A 4-year-old German koi has one eye bulging out, swims tilted to one side, cannot control its buoyancy and hangs tilted near the surface. Its belly is also slightly swollen. The other fish are normal. Last week heavy rain washed brown water from the neighboring field into the pond.

Which is the most appropriate response?

1. Push the eye back into place with a finger; the tilt is air in the swim bladder, so press the belly to let the air out
2. The effects of the rain will pass with time; leave the water as is and just watch the fish
3. Test the water and change it with new water, isolate the fish in a 0.5% salt bath, and consult a fish vet the same day
4. It looks hard to swim, so lower the pond level to make it shallow and feed more to build strength

AnswerC. Test the water and change it with new water, isolate the fish in a 0.5% salt bath, and consult a fish vet the same day

ExplanationA bulging eye, tilting and a swollen belly indicate a serious condition: fluid buildup from internal disease or bacterial infection, or swim bladder trouble. Runoff from a field may contain pesticides, fertilizer and mud, and water deterioration or poisoning may have triggered it. First test the water and change it with new water, isolate the fish in a 0.5% salt bath to reduce osmotic stress, and consult a fish vet the same day. Pressing the eye or belly damages the eyeball or organs and is fatal. Leaving contaminated water affects the other fish too. Lowering the water level increases temperature swings, and feeding burdens the organs.

IssuesThe pond adjoining a field was left in a layout where heavy rain carries water containing pesticides and fertilizer into it. Water was not tested after the inflow, and the problem was only noticed a week later from severe symptoms. Incorrect physical measures such as pressing the eye or belly were being considered.

ImprovementBuild a bank or ditch around the pond so rainwater and field runoff cannot flow in. Lower the water level ahead of typhoon or heavy-rain forecasts, and always test and change the water after any inflow. Check the neighboring field's spraying dates and cover the pond with a sheet during spraying. Look up a fish vet's contact details.

At 6 a.m. on a June morning, you go into the garden and find a 40 cm koi lying on the ground 1 m from the pond. Its body is still slightly moist and the gill covers are moving faintly. There is only 5 cm between the pond rim and the water surface, and a cat was walking through the garden last night. No anti-jump net is fitted.

Which is the most appropriate first action?

1. Wipe the body with a dry towel before returning it to the water, then chase it with a net to make it swim
2. Assume it cannot be saved and bury it as is
3. Quickly run tap water directly over it to wet the body, then toss it into the pond
4. Lift it gently with wet hands or a wet towel, return it to aerated pond water at the same temperature, and rest it in a 0.5% salt bath

AnswerD. Lift it gently with wet hands or a wet towel, return it to aerated pond water at the same temperature, and rest it in a 0.5% salt bath

ExplanationIf the gill covers are moving even faintly, there is a chance of survival. A fish's body is protected by a mucus layer; touching it with dry hands or a towel strips the mucus and invites bacterial infection. Lift it gently with wet hands or a wet towel, return it to aerated pond water at the same temperature (ideally an isolation container), reduce osmotic stress with a 0.5% salt bath and let it recover in the dark and quiet. Wiping with a dry towel damages the mucus, and chasing is fatal to a weakened fish. Burying a fish whose gills are moving is far too early. Cold chlorinated tap water damages the gills and mucus, and the impact of tossing is dangerous. If the body turns white or it stops moving, consult a fish vet the same day.

IssuesThe 5 cm gap between the water and the rim was too small, and no anti-jump net was installed despite koi's habit of leaping when startled. The environment let night predators such as cats approach the pond. The owner did not know the correct handling of a jumped-out fish (wet hands, protecting the mucus).

ImprovementKeep 20 cm or more between the surface and the rim and keep an anti-jump net over the whole pond. Install netting or motion-sensor lights to keep cats and raccoons away at night. Place an isolation container, air pump and pre-measured salt beside the pond and share the jump-out procedure with the family. Make a morning check a routine.

In August, a small koi (20 cm) is stuck against the intake of the pond's submersible pump, tail fin sucked in and struggling. No guard is fitted. Looking in a panic, part of the tail fin is torn and bleeding red, and the fish swims unsteadily after coming free. The water is 28°C and there are 10 other koi in the pond.

Which is the most appropriate response?

1. Switch off the pump before freeing the fish, rest it in an isolation container with a 0.5% salt bath, and consult a fish vet if bleeding continues
2. Pull the fish off by hand with the pump still running, apply styptic to the wound and return it to the pond
3. Bleeding underwater is no problem. Think about a guard later and let it keep swimming
4. Glue the torn fin together with superglue and leave the pump off to prevent a recurrence

AnswerA. Switch off the pump before freeing the fish, rest it in an isolation container with a 0.5% salt bath, and consult a fish vet if bleeding continues

ExplanationPump intakes are a major hazard for fish; first cut the power to remove the suction, then free the fish. Pulling it off while running tears the fins or body. Fin bleeding usually stops naturally in clean water, so rest the fish in an isolation container with a 0.5% salt bath to prevent secondary bacterial infection, in the dark and quiet. If bleeding does not stop or the wound turns cloudy white, consult a fish vet. Styptics and glue damage the mucus and do not work underwater. Leaving the pump off stops oxygen supply and filtration, endangering every fish in the pond, especially in summer. The right course is to fit a guard and then restart.

IssuesThe owner did not know fish could be sucked into the pump intake and had not fitted a guard. There was no risk assessment for keeping small fish with a powerful pump in the same pond, and no basic procedure of cutting the power first in an accident.

ImprovementAlways fit a guard, sponge or mesh to the intake, and in ponds with small fish use a large-diameter intake with gentle suction. Make sure every family member knows where the pump switch is and cut the power before responding to an accident. Keep a spare air pump so oxygen is maintained while the pump is off.

In April, to clean the pond, you caught a 50 cm koi with a coarse nylon net. The fish thrashed violently, and when taken out several scales on its flank had come off, with slight bleeding from the reddened area. The surrounding mucus also looks white and peeled. The water is 16°C, and you are just about to start cleaning the pond.

Which is the most appropriate response?

1. Apply human antiseptic to the bare area, wait a few minutes for it to dry, then return the fish to the pond
2. Scales do not grow back, so for appearance's sake shave the bare area flat
3. Rest it in an isolation container with a 0.5% salt bath; if the wound swells white or spreads red, see a fish vet within a few days
4. A few scales are no problem. Return it to the pond right away and feed plenty to speed recovery

AnswerC. Rest it in an isolation container with a 0.5% salt bath; if the wound swells white or spreads red, see a fish vet within a few days

ExplanationLost scales and damaged mucus are an entry point for bacterial infections such as hole-in-the-side disease, which is common at 15–20°C. An isolation container with a 0.5% salt bath reduces osmotic stress and limits bacterial growth while the mucus regenerates. If the wound swells white or spreads red, infection has begun: consult a fish vet within a few days. Human antiseptic damages the mucus, and time in the air dries the mucus and gills. Scales regenerate, so there is no need to shave them; it only widens the wound. Returning it straight to a bacteria-rich pond and feeding raises the risk of fouling and infection.

IssuesA large fish was caught with a coarse nylon net, and its thrashing scraped the body. The owner did not know catching was done in the temperature band where hole-in-the-side disease is common and infection risk is high. The catching method and tools were not designed to protect the body surface.

ImprovementSwitch to a rubber net or guide the fish underwater into a plastic bag or tub so the body is not scraped. Limit cleaning and moves to a few times a year and avoid the 15–20°C spring and fall period where possible. Always check the body surface after catching, and keep an isolation container and salt ready.

In August, three koi are kept in a shallow concrete pond 40 cm deep. It gets direct sun during the day and the water reaches 33°C. For several days the color on their backs has been fading, and today the skin on the back is red, raw and peeling white. The fish are sluggish in the sunny area near the surface. There is no shade or floating plants.

Which is the most appropriate response?

1. Apply human sunscreen or ointment to the sores and keep the koi in an indoor bucket for a few hours to dry
2. Immediately put up shade netting to create shade and boost aeration. Rest the fish in a 0.5% salt bath and see a vet if the sores spread
3. Dump large amounts of ice into the pond to cool it to 25°C at once and leave it in the sun
4. It is natural summer fading; leave it and increase color-enhancing food to restore the color

AnswerB. Immediately put up shade netting to create shade and boost aeration. Rest the fish in a 0.5% salt bath and see a vet if the sores spread

ExplanationIn a shallow pond in direct sun, a koi's back gets sunburned by UV and high temperature, and the raw skin becomes an entry point for bacterial infection. First create shade with netting and make up for the oxygen lost at high temperature with aeration. Put the fish in an isolation container with a 0.5% salt bath, and consult a fish vet if the sores spread red or swell. Human medications and drying in the air damage the mucus and gills and are fatal. Cooling rapidly with ice causes temperature shock (guide: within 2°C per day). Dismissing the fading as natural lets the sores expand and progress to hole-in-the-side disease.

IssuesA 40 cm-deep pond with no shade created a harsh environment of 33°C in summer sun. The owner did not understand that koi need a deep area of 1 m or more and ignored the early sign of fading on the back.

ImprovementCreate shade with shade netting, a sunshade or floating plants, and if possible add a deep area of 1 m or more. In summer check the thermometer daily; above 30°C add shade, boost aeration and reduce feeding. Check back color and skin condition every morning and consult a fish vet early if anything is wrong.

During May spawning season, a female koi being chased by a male crashed hard into the concrete edge of the pond. The area around her mouth is torn and bleeding, and a barbel on one side has been ripped off. She continues to be chased and drifts at the surface with her mouth open. The pond is lined with angular stones.

Which is the most appropriate response?

1. Take the fish out of the water to examine the mouth, spreading it open with tweezers to check inside
2. It is a spawning-season event, so let nature take its course and leave her in the pond even if chased
3. Reattach the torn barbel with superglue and apply human ointment to the wound
4. Move the female to a separate container away from the chasing and rest her in a 0.5% salt bath. Consult a fish vet if bleeding continues

AnswerD. Move the female to a separate container away from the chasing and rest her in a 0.5% salt bath. Consult a fish vet if bleeding continues

ExplanationIn spawning season males chase females hard, and mouth and body injuries from collisions increase. Left in place, an injured female keeps being chased, the wound worsens and she weakens. First move her to a separate container away from the chasing and rest her in a 0.5% salt bath to control secondary infection. Torn mouths and damaged barbels usually heal naturally underwater, but if bleeding continues or the wound turns cloudy white, consult a fish vet. Taking the fish out and spreading the mouth dries the mucus and gills and is extremely dangerous. Leaving her leads to weakening and bacterial infection. Glue and human ointment do not work underwater and are harmful; barbels usually regrow.

IssuesMales and females were kept densely in the same pond during spawning season, and the risk of injury from chasing was ignored. The pond edge and stones were angular, a structure that causes serious injuries on impact. There was no thought of isolating an injured fish from the chasing.

ImprovementIn spring spawning season, separate the sexes or move females to a separate container while chasing is intense. Round off the pond edge and stones and remove protruding pipes to reduce impact injuries. Prepare an isolation container, air pump and salt before spawning season, and check mouths and fins every morning.

In October, while cleaning with the pond almost drained, you failed to notice a 30 cm koi left in the murky bottom and stepped on it with a rubber boot. The fish is on its side, belly up, moving its gill covers rapidly. Some scales on one side have come off with slight bleeding. The other fish had already been moved to a tub.

Which is the most appropriate response?

1. Lift it gently with wet hands, move it to an isolation container of aerated pond water at the same temperature, rest in a 0.5% salt bath and consult a fish vet the same day
2. It is belly up, so turn it over and shake it up and down in the water repeatedly to bring it round
3. It may have broken bones, so splint the body with a board and bandage and return it to the pond
4. Decide it cannot be saved once stepped on and prioritize the other fish

AnswerA. Lift it gently with wet hands, move it to an isolation container of aerated pond water at the same temperature, rest in a 0.5% salt bath and consult a fish vet the same day

ExplanationBeing stepped on is a serious accident that can cause organ damage or fractures, but if the gills are moving there is a chance of survival. Lift gently with wet hands (to protect the mucus), move to an isolation container of aerated pond water at the same temperature, rest in a 0.5% salt bath in the dark and quiet, and consult a fish vet the same day. Shaking worsens internal injuries and hastens death from stress. A board or bandage damages the mucus, cannot hold underwater and interferes with breathing. Giving up on a fish whose gills are moving is far too early. If it stays on its side for hours and gill movement stops, recovery is unlikely.

IssuesThe procedure of moving every fish to a tub before draining was not followed, and footing was not checked in murky water. There was no checklist to confirm no fish remained before starting work. The severity of internal injury from being stepped on tended to be underestimated.

ImprovementBefore cleaning, always move every fish to a tub, count them and confirm before draining. While draining, visually check for fish while the water is still shallow and clear, and feel with a hand before placing a foot on the bottom. Prepare an isolation container, air pump and salt on cleaning day and confirm the fish vet's contact details before starting.

One March morning a grey heron was perched over the pond. After chasing it off, you find one koi with a deep stab wound on its back, scales lifted and red flesh showing. The fish is swimming but leaning to one side. The pond has no net or fishing line over it. The water is 12°C.

Which is the most appropriate response?

1. The wound will heal naturally underwater, so do nothing and think about heron measures another day
2. Rub human wound ointment into the stab and keep the fish in a bucket on land until the wound closes
3. Prevent infection in an isolation container with a 0.5% salt bath, see a vet within a few days if the wound worsens, and install a heron net over the pond the same day
4. To keep bacteria out of the wound, add strong salt water and fish medication to the whole pond at once

AnswerC. Prevent infection in an isolation container with a 0.5% salt bath, see a vet within a few days if the wound worsens, and install a heron net over the pond the same day

ExplanationHeron stab wounds are deep, heal slowly in cold water and easily become infected (hole-in-the-side disease and the like). Reduce osmotic stress and control infection in an isolation container with a 0.5% salt bath, and consult a fish vet within a few days if the wound swells white or spreads red. Once a heron learns a feeding spot it returns daily, so install a net or fishing line the same day to prevent another attack. Doing nothing lets the deep wound become infected, and the heron will attack other fish. Human ointment harms the mucus, and a bucket on land dries the gills and is fatal. Strong salt plus medication in the whole pond cannot be controlled and harms the filter bacteria and other fish.

IssuesThe pond had no heron net or fishing line, allowing predator access. The owner did not know herons come at dawn and return once they have learned a spot, and put off countermeasures. Incorrect measures such as keeping the injured fish on land or medicating the whole pond were considered.

ImprovementNet the whole pond or stretch fishing line in a grid overhead to stop herons landing. Install shade boards or pipes in the pond as hiding places. Patrol the pond at dawn and dusk and check for predator traces (footprints, feathers). Keep an isolation container, salt and the fish vet's contact details ready.

In February, a 1-year-old koi in an indoor 120 cm tank was startled during tank cleaning, leaped and hit its eye on a glass corner. The surface of one eye is cloudy white and slightly red around it. The fish can swim but tends to bump into obstacles on that side. The water is 20°C and water quality is normal.

Which is the most appropriate response?

1. Wipe the cloudiness off the eye with gauze, apply human eye drops directly and return it to the tank
2. Keep the tank dark and quiet and prevent infection with a 0.5% salt bath. Consult a fish vet if no improvement in a few days
3. The eye will not heal, so give the fish away to someone else for appearance's sake
4. Remove all the decor so it cannot bump into anything and do a full water change to avoid any further impact

AnswerB. Keep the tank dark and quiet and prevent infection with a 0.5% salt bath. Consult a fish vet if no improvement in a few days

ExplanationCorneal clouding and surrounding redness from a knock are traumatic inflammation and usually recover with clean water and rest. Keep the tank dark and quiet to reduce stress and prevent secondary bacterial infection with a 0.5% salt bath. If the clouding does not clear in a few days, the eye bulges or the redness spreads, consult a fish vet. Wiping with gauze damages the cornea, and human eye drops are of unknown safety for fish and do not work underwater. Traumatic clouding usually heals, so there is no need to give up. Removing the decor and a full water change are a drastic environmental change that stresses the fish and loses filter bacteria.

IssuesThe owner did not anticipate the fish leaping in fright during cleaning and worked with the lid and net removed. The risk of a startled fish hitting a glass corner was not considered and nothing was done to keep the fish calm during work. Incorrect physical treatment of the eye injury was being considered.

ImprovementLeave part of the lid or net on during cleaning, and avoid sudden movements or loud noises. Pad the corners of glass tanks with cushioning. Check eyes and fins at daily feeding, and keep a 0.5% salt bath and the fish vet's contact details ready. Make environmental changes gradually, not all at once.

In July, decorative pebbles laid along the pond edge had fallen into the pond. A 35 cm koi that was poking the bottom now has its mouth wide open and cannot close it, trying repeatedly to spit something out without success. Looking into the mouth, a pebble is lodged there. The fish moves its gills rapidly and looks distressed.

Which is the most appropriate response?

1. Take the fish out of the water, pull the stone out forcefully with pliers and return it to the pond when done
2. It will come out on its own eventually; leave it in the pond and check again tomorrow
3. Blast the mouth with tap water to flush the stone out, and if it does not come out, push it further in
4. Restrain the fish underwater with wet hands; if it cannot be removed easily, do not force it and consult a fish vet the same day

AnswerD. Restrain the fish underwater with wet hands; if it cannot be removed easily, do not force it and consult a fish vet the same day

ExplanationKoi habitually suck up bottom sand and mud, so pebbles small enough to fit in the mouth placed in the pond can be swallowed by mistake, and one lodged in the mouth prevents breathing and risks suffocation. Restrain the fish underwater with wet hands, and if the stone is visible and looks easy to remove, take it out gently. If not, do not force it; consult a fish vet the same day. Taking the fish out and pulling with pliers damages the mouth lining and pharynx and dries the gills. Left alone, it dies of respiratory failure. Tap water damages the mucus with chlorine and temperature difference, and pushing the stone in lodges it in the pharynx, which is fatal.

IssuesDecorative stones small enough to fit in the mouth were placed along the pond edge, with no risk assessment of koi's bottom-poking habit. Nothing prevented the pebbles from falling into the pond. A dangerous forced removal of the foreign object was being considered.

ImprovementDo not place pebbles or ornaments small enough for a koi's mouth around or in the pond, and keep the bottom bare or concrete. Tidy the feeding area so torn food-bag pieces and plastic scraps are not dropped in. Learn how to restrain a fish underwater and post the fish vet's contact details beside the pond.

In September a large typhoon is approaching with heavy rain forecast from tonight. The pond has an overflow pipe but no screen, and in a previous downpour the water rose to the rim. Downstream of the pond, a gutter leads to a river. There are 20 koi and two large tubs for evacuation. The rain has just started.

Which is the most appropriate first action?

1. Lower the water level now, stretch netting over the overflow pipe and pond rim to prevent escape, and prepare a battery air pump in case of a power outage
2. Rain brings fresh water and the fish will enjoy it, so do nothing and leave the pond to itself
3. When the water reaches the rim, move all the koi into the tubs and keep them in the entrance hall overnight
4. They can survive in the river even if washed out, so do not net the pond and let the fish do as they please

AnswerA. Lower the water level now, stretch netting over the overflow pipe and pond rim to prevent escape, and prepare a battery air pump in case of a power outage

ExplanationIf koi wash out of the pond in typhoon flooding, they flow via the gutter into the river, which is illegal release and may spread disease into the wild. The correct course is to lower the water level before the rain intensifies, net the overflow pipe and rim to prevent escape, and prepare a battery air pump against a power outage. Rainwater abruptly changes pond water quality and carries in mud and pesticides, so the idea that fish enjoy it is wrong. Twenty fish in two tubs overnight means overcrowding, oxygen depletion and fouled water, plus heavy stress from the move. Escape is the owner's responsibility, and releasing into rivers is prohibited.

IssuesDespite past experience of water reaching the rim, no overflow screen or escape-prevention net had been installed. There was no set typhoon preparation procedure (water level, outage measures), and little awareness that escape counts as illegal release.

ImprovementKeep a screen on the overflow pipe at all times and keep the rim at least 20 cm above the water. Record on the calendar the procedure of lowering the water level the day before a typhoon or heavy-rain forecast. Keep a battery air pump and spare batteries on hand and practice maintaining oxygen during an outage. Test and change the water after heavy rain.

In June, when you opened the bottom drain valve for a partial water change, several 5 cm fry born in spring were sucked into the current and went into the unscreened drain opening. You hurriedly closed the valve, and looking into the drain pit at the end of the pipe, the fry are moving in the muddy water.

Which is the most appropriate response?

1. Let the muddy water in the pit flow on into the gutter and leave the fry to survive in the wild
2. Dump the muddy water from the pit back into the pond with a bucket, fry and all
3. Scoop the fry from the pit with a soft net, move them to a container of pond water at the same temperature with aeration, and return them to the pond once recovered
4. The fry are tiny, so give up on them, reopen the drain and finish the water change

AnswerC. Scoop the fry from the pit with a soft net, move them to a container of pond water at the same temperature with aeration, and return them to the pond once recovered

ExplanationThe fry in the drain pit are still alive, so scoop them gently with a soft net, move them to a container of pond water at the same temperature, supply oxygen with aeration, and return them to the pond once they have settled. Pouring the pit's muddy water back brings in sludge and bacteria, and letting it flow to the gutter washes the fry into the river, which is illegal release and affects the ecosystem. Reopening the drain washes the fry down the gutter to die or escape. The accident happened because the drain had no screen; always fit a guard before changing water from now on.

IssuesThe drain had no screen or guard, and the risk of fry being sucked in was not anticipated. In a pond where fry had hatched in spring, water was changed with the same procedure as for adults, ignoring the small fish. There was insufficient awareness that letting the pit water flow on amounts to release.

ImprovementAlways fit fine mesh or guards to drains and intakes, and move fry to a separate container before water changes when they are present. Open drains slowly rather than all at once, watching how the fish move. Install a screen in the drain pit as well to prevent any escape. Keep a soft fry net and small containers on hand.

At 10 p.m. on an August night, lightning caused a local power outage. The pond pump, aeration and waterfall have all stopped, and the water is 29°C. There are 15 koi and no idea when power will return. On hand are one battery air pump and two jerry cans of stored (dechlorinated) water. The fish are still swimming calmly.

Which is the most appropriate first action?

1. The fish are calm, so do nothing until morning and check after power is restored
2. Set up the battery air pump immediately, do a partial water change with the stored water to add oxygen, and check regularly until morning for piping
3. Add lots of ice to lower the temperature and feed them at the same time to keep their strength up
4. Move them all to the indoor bathtub, fill it with tap water and keep them there until morning

AnswerB. Set up the battery air pump immediately, do a partial water change with the stored water to add oxygen, and check regularly until morning for piping

ExplanationOn a hot night dissolved oxygen is at its lowest, and once the pump stops, oxygen depletion begins within hours. First set up the battery air pump to add oxygen, then do a partial change with the stored dechlorinated water to improve oxygen and water quality. Check regularly until morning for piping and add more water changes if there are signs of oxygen shortage. Even if the fish look calm, oxygen depletion accelerates sharply before dawn, so waiting is dangerous. Dumping in ice causes temperature shock, and feeding during an outage raises oxygen demand and fouls the water. Moving 15 large fish to a bathtub means overcrowding and oxygen shortage, and the chlorine and temperature difference of tap water damage the mucus.

IssuesOutage preparation was relatively good, with a battery air pump and stored water, but one air pump was not enough for 15 fish. There was insufficient awareness that a summer-night outage is when oxygen depletion is most likely, and the outage priorities (secure oxygen, stop feeding) were not clear.

ImprovementKeep several battery air pumps sized to the pond and check spare batteries each season. Always keep dechlorinated stored water on hand. Post a written outage procedure (set up air, change water, stop feeding, check regularly). Consider a small generator or portable power station if possible.

In September, the extension cord for the underwater lighting in the garden pond was lying in a rain puddle. Looking into the pond, several koi are swimming erratically as if convulsing and going rigid at the surface. The cord's connector is in the water at the pond edge, and the moment you reached toward the pond water you felt a slight tingling in your hand.

Which is the most appropriate first action?

1. Quickly put your hands in the pond and scoop out all the convulsing fish
2. Pull the cord out by hand to cut the power, then rescue the fish
3. The thrashing fish are dangerous, so stay away from the pond and wait for the water to recede
4. Without touching the pond or water, cut the power at the breaker or the outlet, confirm it is safe, then move the fish to an aerated container

AnswerD. Without touching the pond or water, cut the power at the breaker or the outlet, confirm it is safe, then move the fish to an aerated container

ExplanationWhen pond water is electrified, a person can be electrocuted and killed. First cut the power at the breaker or the outlet without touching the pond or water, confirm it is safe, and then rescue the fish. Putting hands in electrified water or pulling a wet cord by hand carries a high risk of electrocution, and if the rescuer collapses the fish cannot be saved either. Cutting the power stops the leak, so there is no need to wait for the water to recede; the sooner the fish are rescued, the better their chance of recovery. Move rescued fish to an aerated container of pond water at the same temperature, and if convulsions or paralysis continue or they lie on their side without righting, consult a fish vet the same day.

IssuesA non-waterproof extension cord and connector were placed where they could be submerged at an outdoor pond, with no ground-fault (earth-leakage) breaker. The owner did not know the basic rule of cutting power first in an electrical accident and endangered themselves by reaching in.

ImprovementAlways connect outdoor pumps and lights to a waterproof outlet with a ground-fault breaker, and keep connectors in a waterproof box above ground level. Route cords away from the pond edge and inspect wiring regularly. Make sure every family member knows where the breaker is and share the rule: if the water feels wrong, do not touch it, cut the power.

On a July afternoon, a 6-year-old playing by the pond picked up a 45 cm koi that had come to hand-feed, holding it in both hands, and dropped it onto the concrete when it struggled. The koi bounced on the ground, scraping its belly, and the mucus there has peeled white. Crying, the child grabbed the koi with dry hands and threw it into the pond. The koi is lying on its side on the pond bottom.

Which is the most appropriate response?

1. It is back in the pond so there is no problem. Scold the child and leave it at that
2. Move it gently with wet hands to an aerated isolation container, rest in a 0.5% salt bath, and see a fish vet the same day if it stays on its side
3. It is on its side, so poke it repeatedly with a net to make it swim and feed it to perk it up
4. It is a bruise, so drop the pond temperature sharply to reduce inflammation and watch until tomorrow

AnswerB. Move it gently with wet hands to an aerated isolation container, rest in a 0.5% salt bath, and see a fish vet the same day if it stays on its side

ExplanationThe impact of the fall may have caused organ damage or fractures, and being grabbed with dry hands stripped a wide area of mucus, so the risk of bacterial infection is also high. Move it gently with wet hands to an isolation container of aerated pond water at the same temperature, give a 0.5% salt bath, and rest it in the dark and quiet. If it stays on its side or gill movement is weak, consult a fish vet the same day. Simply returning it to the pond leaves a weakened fish to be chased by others and infected in dirty water. Poking with a net adds stress and injury, and feeding burdens the organs. A sudden temperature drop causes temperature shock and does nothing for inflammation.

IssuesSupervision was inadequate in an environment where a child could lift a fish. The child had not been taught that koi, though tame enough to hand-feed, are not to be picked up. The family had not shared the basic fact that dry hands strip a fish's mucus.

ImprovementTeach children not to lift koi and to touch only with wet hands, and have adults present at feeding. Post a sign at the pond edge: do not lift the fish. Keep an isolation container, air pump and salt beside the pond, and write out and share the procedure for a fall (wet hands, isolation, salt bath, criteria for seeing a vet).

At midday in May, you put a hose in the pond to top it up and forgot it for two hours. By the time you noticed, the pond water had been almost entirely replaced with tap water and the temperature had dropped from 20°C to 13°C. The koi are sitting still on the bottom; several are shivering slightly and moving their gill covers rapidly. No dechlorinator was added.

Which is the most appropriate first action?

1. Pour boiling water into the pond to bring the temperature straight back up to 20°C
2. The water has already been replaced, so do nothing and see how they are tomorrow
3. Scoop out all the fish into buckets of stored water, dry out the pond completely and rebuild it
4. Stop the hose, immediately add the labeled dose of dechlorinator to the whole pond and boost aeration. Do not heat rapidly; consult a fish vet the same day

AnswerD. Stop the hose, immediately add the labeled dose of dechlorinator to the whole pond and boost aeration. Do not heat rapidly; consult a fish vet the same day

ExplanationChlorine directly damages the gills and mucus, and a sudden 7°C drop is also severe stress. First stop the hose, add the labeled dose of dechlorinator to the whole pond to neutralize the chlorine, and boost aeration to help breathing. Do not restore the temperature quickly; let it return naturally (guide: within 2°C per day). Shivering and rapid breathing are signs of chlorine and cold shock, so consult a fish vet the same day. Boiling water creates local hot spots and another sudden change, which is fatal. Left alone, chlorine destroys the gills and fish may die within hours. Moving every fish to buckets causes overcrowding and oxygen shortage, and rebuilding the pond loses the filter bacteria.

IssuesThe hose was left unattended with no timer or reminder. The owner underestimated that undechlorinated tap water damages gills with chlorine and the risk of a sudden temperature drop. There was no set top-up procedure (amount, dechlorinator, temperature).

ImprovementAlways add dechlorinator before topping up, set a kitchen timer and do not leave the hose. Limit top-ups to 20–30% of pond volume at a time with a temperature difference within 2°C. Label the dechlorinator dose on the container and keep it in stock. If possible, fit an automatic shut-off timer to the tap.

Late one March night, a noise wakes you and you find a 25 cm koi has jumped through a gap in the lid of the 120 cm tank and is flopping on the floor. The floor is hardwood, and dust and hair are stuck to the koi's body. Its gills are moving. There are three other fish in the tank, and the feeding hatch in the lid had been left open.

Which is the most appropriate response?

1. Lift it with wet hands, rinse the dust off lightly in a container of tank water, then rest it with aeration in a 0.5% salt bath
2. It is dirty with the dust on it, so wash its body with soap, wipe it with a towel and put it back
3. Do not touch it while it is flopping; once it settles, toss it back into the tank with dry hands
4. Rinse the body vigorously under the tap, return it to the tank and feed it

AnswerA. Lift it with wet hands, rinse the dust off lightly in a container of tank water, then rest it with aeration in a 0.5% salt bath

ExplanationA jumped-out fish is starting to dry out, and floor dust and hair are stuck to its body. Lift it gently with wet hands, rinse lightly in a container of tank water to remove debris, then reduce osmotic stress with aeration and a 0.5% salt bath and rest it in the dark and quiet. Soap dissolves the mucus and a towel strips it; both are strictly forbidden. Leaving it flopping is fatal from drying and impact, and tossing it back with dry hands causes mucus damage and impact. Tap water damages the mucus with chlorine and temperature difference, and feeding right after is a burden. If it still has clamped fins or lies on its side the next day, consult a fish vet.

IssuesThe feeding hatch was left open, creating a gap for jumping out. The owner did not understand that koi leap when startled and are especially prone to jumping at night with noises or sudden lighting changes. The jump-out procedure (wet hands, rinse, salt bath) had not been shared with the family.

ImprovementClose the tank lid with no gaps and make a habit of shutting the feeding hatch right after feeding. Turn lights off gradually at night and avoid loud noises near the tank. Keep an isolation container, air pump and salt beside the tank and post the jump-out procedure. Move tank-kept koi to a pond or large tank as they grow.

At midday in August, you are transporting a 50 cm koi given to you by an acquaintance, in a bag of pond water and air on the back seat of the car. Traffic has made the trip two hours and the car is hot. Looking in the bag, the koi is gasping at the surface, the water is warm and starting to turn cloudy white. Home is another 40 minutes away.

Which is the most appropriate first action?

1. You are in a hurry, so speed up without jostling the bag and head straight home
2. Open the bag and pour in cold water bought at a convenience store to drop the temperature at once
3. Stop somewhere safe, open the bag and refill it with fresh air, cool the car with air conditioning and keep the bag out of direct sun. Use a battery air pump if possible
4. Take the koi out of the bag into a bucket in the car and drive with the windows open

AnswerC. Stop somewhere safe, open the bag and refill it with fresh air, cool the car with air conditioning and keep the bag out of direct sun. Use a battery air pump if possible

ExplanationThe oxygen in a bag is limited; at high temperature dissolved oxygen falls and the fish's metabolism rises, so two hours in traffic has brought on oxygen depletion and fouled water. Stop safely, open the bag, refill it with fresh air and reseal, cool the car with air conditioning and keep the bag out of direct sun. If you have a battery air pump, put it in the bag to add oxygen. Driving on risks suffocation within 40 minutes. Pouring in cold water causes shock from the sudden temperature change and contains chlorine. A bucket has no lid, water sloshes with the car's motion and the fish can jump out, and hot air from the windows makes things worse. On arrival acclimate carefully, and consult a fish vet if the fish lies on its side.

IssuesThe fish was moved in midsummer in just a bag, without anticipating traffic or heat. Basic transport equipment such as a foam box, cooling and a battery air pump was lacking. The owner did not understand that bag oxygen lasts only a short time and even less in heat.

ImprovementTransport fish with the bag inside a foam box, ice packs in the corners of the box, kept dark to hold the temperature. Fill with oxygen if oxygen packing is available and carry a battery air pump. In summer travel in the cool of early morning or evening and choose routes with less traffic. Prepare the acclimation and quarantine container before departure.

One November morning, the pond level has dropped to less than half overnight. The liner seems to have a hole and the ground around is wet. The water is 30 cm deep, the 10 koi have their backs near the surface and are sluggish. The water is 10°C. On hand are three large tubs and no stored water.

Which is the most appropriate first action?

1. Fill the pond straight back up to the top with tap water from a hose
2. Move the koi into the tubs using pond water, spread out with aeration, refill the pond gradually using dechlorinator, and arrange repair of the hole at the same time
3. Koi can survive in shallow water, so leave them as they are for a few days until the repair contractor comes
4. Bag up all the koi and take them to an acquaintance's pond to board until the repair is done

AnswerB. Move the koi into the tubs using pond water, spread out with aeration, refill the pond gradually using dechlorinator, and arrange repair of the hole at the same time

ExplanationAt 30 cm the fish's backs are nearly exposed, leaving them vulnerable to daytime temperature swings and predators. First use the remaining pond water to house the koi spread across the tubs, securing oxygen with aeration. Refill the pond gradually with dechlorinator, keeping the temperature difference small, while arranging the repair. Filling rapidly with tap water harms the fish with chlorine and a sudden temperature change. Leaving them for days in shallow water is fatal from temperature swings, oxygen shortage and heron attacks. Moving to an acquaintance's pond adds transport stress and risks exchanging pathogens such as KHV between ponds, so it should be avoided as a rule.

IssuesThe liner was not inspected regularly for wear or damage, and the sudden drop was not noticed until morning. No stored or spare water was kept, and the emergency housing plan (number of tubs, air pumps) was insufficient. There was little awareness that boarding fish at another pond can spread infection.

ImprovementInspect the liner and pipes each season, mark the water level and check it every morning. Keep dechlorinated stored water on hand, enough for an emergency top-up of about 20% of pond volume. Keep tubs and air pumps sized to the number of koi, and keep to the rule of not exchanging fish or water with other ponds.

On a June afternoon, right after the neighbor sprayed pesticide on their garden trees, the koi in the downwind pond are thrashing wildly and several have rolled over and are convulsing at the surface. A thin oily film floats on the water. The pond is open on top and holds 2 tons of water. There are 500 L of stored water.

Which is the most appropriate first action?

1. The thrashing is excitement, so give food to calm them and watch until tomorrow
2. Scoop out only the fish that have rolled over, leave the pond water as is and shelter them indoors
3. Add plenty of activated carbon to the pond, leave it for a few hours and wait for the film to disappear
4. Immediately remove the surface film, do a large water change with stored water, maximize aeration, and contact a fish vet and the prefectural fisheries division

AnswerD. Immediately remove the surface film, do a large water change with stored water, maximize aeration, and contact a fish vet and the prefectural fisheries division

ExplanationPoisoning from pesticide drift can be lethal within minutes to hours, so quickly skim the surface film, do a large water change with stored dechlorinated water to dilute the pesticide, and maximize aeration to help breathing. At the same time consult a fish vet and ask the neighbor for the product name to identify the substance. If many fish die, consult the prefectural fisheries division and do not release dead fish or water into a river. The thrashing is a neurological symptom of poisoning and will not settle with food. Moving only the rolled-over fish leaves the rest to keep absorbing poison. Activated carbon alone cannot lower the concentration fast enough, and waiting leads to death.

IssuesThe owner did not know the neighbor's spraying schedule and left the open-topped pond unprotected. The risk of pesticide drifting into the pond on the wind was not anticipated and nothing was ready to cover the pond during spraying. Stored water for emergency changes was only a quarter of the pond volume, insufficient for an emergency.

ImprovementAsk neighbors to give advance notice of pesticide spraying, and on those days cover the pond with a sheet and protect the pump intake. Keep dechlorinated stored water equal to about half the pond volume for emergency changes. Post the contact details of a fish vet and the prefectural fisheries division and share the poisoning procedure with the family.

In April you built a new concrete pond in the garden, filled it a week after completion and immediately put in five koi. The next morning a white powdery film floats on the surface, the koi are gasping at the surface, and the mucus on their bodies is starting to peel white. The pH measures over 10. The pond was not leached (cured) before use.

Which is the most appropriate response?

1. Move the koi at once to a separate container of safe pond water with aeration, drain the pond and redo the leaching. Rest fish with peeled mucus in a 0.5% salt bath
2. Add a large dose of pH-lowering chemical at once to neutralize it and leave the fish in the pond to watch
3. The white film will soon disappear; wait a few days without changing water and the koi will adjust
4. The koi's problem is the food, so change the food and leave the pond water as is

AnswerA. Move the koi at once to a separate container of safe pond water with aeration, drain the pond and redo the leaching. Rest fish with peeled mucus in a 0.5% salt bath

ExplanationNew concrete leaches alkaline compounds, producing strongly alkaline water with a pH above 10. This chemically damages the koi's mucus and gills, and left alone they die within days. First move the koi to a separate container of safe water with aeration, and rest fish with peeled mucus in a 0.5% salt bath to prevent infection. Drain the pond and redo a thorough leaching (fill and replace the water several times until pH stabilizes). Dropping pH rapidly with chemicals causes further shock from the sudden swing and does not stop the underlying leaching. The white film is evidence of leaching; it will not clear in days and the koi will not adjust. The food is not the cause.

IssuesThe new concrete pond was not leached and koi were added a week after completion. The owner lacked the basic knowledge that concrete leaches alkali and raises pH, and did not test pH before adding fish. A safe holding container and water were not adequately prepared.

ImprovementAfter completing a concrete pond, fill and replace the water several times and confirm pH is stable at 7–8 before adding fish. Always test pH, ammonia and nitrite in a new pond or tank beforehand. Trial-run with a few fish for several weeks until filter bacteria establish. Prepare a holding container and air pump in advance.

In July you applied herbicide to the weeds around the garden pond. That night a sudden shower washed rainwater into the pond. The next morning several koi are drifting limply at the surface with weak gill-cover movement. The water is slightly foamy. The pond holds 1.5 tons and there are 300 L of stored water. The other fish are sitting still on the bottom.

Which is the most appropriate first action?

1. The foam is like detergent, so stop the pond pump until the foam clears
2. Add fish medication to kill bacteria and wait for the koi to recover
3. Change as much water as possible with the stored water and maximize aeration, continue stepwise changes with new water afterward, and consult a fish vet the same day
4. Herbicide only affects plants, so it is unrelated. Suspect another disease and observe for a few days

AnswerC. Change as much water as possible with the stored water and maximize aeration, continue stepwise changes with new water afterward, and consult a fish vet the same day

ExplanationSome herbicides contain ingredients or surfactants harmful to fish, and when rain washes them into the pond they cause poisoning. Weak gill movement and limp drifting are a dangerous state: change as much water as possible with stored water to lower the concentration and maximize aeration to help breathing. Continue stepwise changes with dechlorinated new water afterward, consult a fish vet the same day and tell them the product name used. Stopping the pump halts oxygen supply and filtration and makes things worse. This is not a bacterial infection, so fish medication is pointless. The idea that herbicide only affects plants is wrong, and leaving it leads to more deaths. Seal dead fish in bags and do not release pond water into a river.

IssuesHerbicide was used around the pond without considering that rain could wash it in. The owner did not know herbicide is harmful to fish and applied it without checking the rain forecast. Stored water for emergency changes was only 20% of pond volume.

ImprovementDo not use herbicides, insecticides or fertilizer around the pond; weed by hand. If use is unavoidable, cover the pond and only apply on days with no rain forecast. Dig a drainage ditch around the pond to prevent inflow. Keep stored water equal to about half the pond volume and post the fish vet's contact details.

On a June evening, a 5 kg bag of pellets left beside the pond toppled in, split open and spread a huge amount of food across the pond. The koi are feeding frantically and their bellies are swollen to bursting. Large amounts of uneaten food float on the surface, the water is 26°C and there are 15 koi in the pond.

Which is the most appropriate first action?

1. The koi will clean up the leftovers if you leave them until tomorrow, so do nothing in particular
2. Net out all the floating food, fast the fish for several days from tomorrow, boost aeration, test the water and change it
3. Press the koi's bellies by hand to make them regurgitate, and let them eat the rest so nothing is wasted
4. To aid digestion, raise the temperature to 30°C with a heater and give a little extra food

AnswerB. Net out all the floating food, fast the fish for several days from tomorrow, boost aeration, test the water and change it

ExplanationKoi keep eating even when full, and overeating causes indigestion and enteritis. Uneaten food left in the pond rots quickly, and the ammonia spike and oxygen consumption bring oxygen depletion and poisoning by the next morning. First remove all the floating food, fast for several days to rest the gut, boost aeration, and test and change the water. Leaving it leads to water-quality collapse. Pressing the belly damages the organs, and letting them eat the rest worsens the overeating. Sudden heating causes temperature shock and extra food is counterproductive. If swelling persists from the next day, or there are white feces or buoyancy problems, consult a fish vet.

IssuesThe food bag was left in an unstable spot beside the pond with no thought that it could fall in. There was little awareness that koi keep eating when full and that leftovers rapidly degrade water quality.

ImprovementTransfer food to a sealed container and store it indoors in a cool dark place away from the pond. Feed only what is eaten in 5 minutes and remove leftovers on the spot. Share the procedure for a spilled bag with the family (collect food, fast, change water), and keep a water test kit and air pump on hand.

In September, to remove algae from the pond edge, a family member put dish detergent on a sponge and scrubbed the walls. It was not rinsed off fully, foam flowed into the pond, and the koi have begun moving their gills rapidly and drifting at the surface with mouths open. The foam has spread across the whole pond. There are 400 L of stored water; the pond holds 1 ton.

Which is the most appropriate first action?

1. Move the koi to containers of stored water with aeration, remove the foam from the pond and do a large water change, and do not return the fish until the detergent is gone
2. The foam will clear with time, so leave the koi in the pond, stop the pump and wait for the foam to settle
3. Pour a large amount of vinegar into the pond to neutralize it and leave the koi as they are
4. Koi are hardy, so just skim the foam, feed them and watch until tomorrow

AnswerA. Move the koi to containers of stored water with aeration, remove the foam from the pond and do a large water change, and do not return the fish until the detergent is gone

ExplanationThe surfactants in dish detergent destroy the gill membranes and fish rapidly become unable to breathe. First move the koi to containers of stored water with aeration to secure oxygen, remove the foam from the pond and repeat large water changes, and do not return the fish until there is no more foaming. Stopping the pump reduces oxygen, and detergent remains even after the foam clears. Vinegar does not neutralize detergent and the sudden pH swing harms the fish further. Hardy as koi are, they are vulnerable to surfactants, and left until tomorrow most would die. After evacuation, consult a fish vet the same day for any fish still breathing fast or lying on its side.

IssuesThe family did not know household detergent must never be used to clean a pond, and there were no rules on cleaning tools or methods. There was a lack of knowledge that detergent is fatal to gills, and no advance preparation for evacuating the fish.

ImprovementMake it known to every family member, and post it, that ponds and tanks are cleaned with water and a brush only, never detergent, bleach or chemicals. Dedicate cleaning tools to the pond and separate them from other uses. Keep stored water, a holding container and an air pump on hand, and set a procedure for chemical contamination (evacuate, change water, see a vet).

At 2 p.m. on an August day with an air temperature of 38°C, the thermometer in a 60 cm-deep pond reads 34°C. The koi are motionless on the bottom, and some are slowly working their mouths near the surface. Half the pond is in direct sun; the waterfall is running but there is no aeration. A normal amount of food was given at noon.

Which is the most appropriate first action?

1. Dump lots of ice into the pond to bring the koi's body temperature down to 25°C at once
2. Appetite drops in the heat, so give a single large portion of high-nutrition food in the evening instead
3. Create shade with shade netting, add aeration, change small amounts of water with new water close to the same temperature, and cancel the evening feeding
4. The thermometer may be broken, so go and buy a new one

AnswerC. Create shade with shade netting, add aeration, change small amounts of water with new water close to the same temperature, and cancel the evening feeding

ExplanationAbove 30°C, dissolved oxygen falls while the koi's metabolism rises, so oxygen shortage and indigestion are likely. First block direct sun with shade netting, add aeration to supply oxygen, and change small amounts of water with new water close to the same temperature to bring the temperature down gradually. Cancel the evening feeding to prevent fouling. Dumping in ice causes shock from the abrupt change (guide: within 2°C per day) and chills only some of the fish severely. High-nutrition food at high temperature causes indigestion and fouls the water. The fish's condition already shows something is wrong, so act before doubting the thermometer. If piping continues, consult a fish vet the same day.

IssuesA shallow 60 cm pond in direct summer sun had no shade or aeration prepared. The owner did not know oxygen falls at high temperature and fed a normal amount at 34°C. There were no set summer management standards (shade, feeding amount, water changes).

ImprovementIn summer cover the whole pond with shade netting and run aeration 24 hours a day. Above 30°C halve or stop feeding and make small daily changes with water at the same temperature. Consider modifying the pond to add a deep area of 1 m or more. Check the thermometer daily and share the heat-wave procedure with the family.

In January, heavy snow has frozen the garden pond over completely and the ice is 5 cm thick. The pond is 1.2 m deep and the koi are almost motionless on the bottom. The pump is off to prevent freezing. A family member worries that they cannot breathe and is about to break the ice with a hammer. The koi are dormant and not being fed.

Which is the most appropriate response?

1. Break up and remove all the ice with a hammer to get air to the koi
2. Put in a heater to bring the water to 15°C overnight and feed them to build strength
3. Scoop out all the koi and move them to an indoor tank until spring
4. Do not break the ice; open a hole in part of it with hot water or a floating de-icer to allow gas exchange. Let them winter quietly in the deep area and leave them alone until spring

AnswerD. Do not break the ice; open a hole in part of it with hot water or a floating de-icer to allow gas exchange. Let them winter quietly in the deep area and leave them alone until spring

ExplanationWith a 1.2 m deep area, koi lower their metabolism at low temperature and winter quietly even under ice. The problem with total freeze-over is that carbon dioxide and other gases cannot escape, which is solved by opening a hole in part of the ice to allow gas exchange. Pour hot water slowly or use a floating de-icer. Breaking the ice with a hammer sends shock waves through the water that harm the dormant fish, and removing all the ice lets the water get too cold. Rapid heating and feeding cause indigestion and temperature shock in dormant koi. Moving large fish to an indoor tank is dangerous from overcrowding, stress and temperature difference, and disrupts dormancy.

IssuesNo de-icer or means of keeping a hole open was prepared ahead of winter freeze-over. The family did not understand that koi winter quietly in the deep area or the role of a hole for gas exchange, and were considering the shock of breaking the ice.

ImprovementBefore winter, install a floating de-icer or a small aerator (to keep part of the surface moving) so a hole remains in the ice. Keep a deep area of 1 m or more, and do not catch, clean or feed in winter. Share the freeze-over procedure with the family (do not break the ice; open a hole with hot water) and watch quietly until the water warms in spring.

One April night there was an earthquake of seismic intensity 5. The concrete of the garden pond has cracked and water is slowly draining away. A power outage has stopped the pump. There are 12 koi, two tubs, one battery air pump and 200 L of stored water. The family is safe and aftershocks continue.

Which is the most appropriate first action?

1. For the koi's sake, start repairing the pond right now and fill the crack with cement
2. After confirming the family's safety, house the koi spread across the tubs using pond water and secure oxygen with the battery air pump. Watch the water level drop and wait for recovery
3. The water loss cannot be stopped, so release the koi into a nearby river to save them
4. The aftershocks are frightening, so leave the koi and stay away from the pond until tomorrow

AnswerB. After confirming the family's safety, house the koi spread across the tubs using pond water and secure oxygen with the battery air pump. Watch the water level drop and wait for recovery

ExplanationIn a disaster, human safety comes first; once secured, move on to saving the koi. Leaving them in a cracked pond with a falling level becomes dangerous as it gets shallow, so house them spread across the tubs using pond water and secure oxygen with the battery air pump. Watch how fast the level drops, top up with stored water and wait for recovery. Starting repairs at night during aftershocks endangers people, and cement cannot patch a wet crack anyway. Releasing koi into a river is illegal release and spreads disease. Leaving them until tomorrow brings oxygen shortage and temperature swings in the shallow pond, plus predator attacks.

IssuesAgainst the possibility of earthquake damage to the pond, housing was short, with only two tubs for 12 fish and a single battery air pump. There was no set disaster procedure (human safety, housing, oxygen) and release, which is illegal, was being considered.

ImprovementKeep housing containers (such as a collapsible pool) and several battery air pumps sized to the number of koi, and check spare batteries regularly. Keep stored water equal to about half the pond volume. Write out the disaster procedure and share it with the family. Inspect the pond for cracks and check pipes once a year, and note earthquake insurance and repair contractor contacts.

In October, after a week of continuous rain, the pond water is brown and cloudy with a mass of fallen leaves on the bottom. The pH measures a low 6.0, down from 7.5 last month. The koi are sunk on the bottom with fins clamped, and the mucus on several has turned cloudy white. The water is 16°C. The overflow pipe is working normally.

Which is the most appropriate response?

1. Remove the leaves and sludge, and change water stepwise with new water at the same temperature to bring pH back gently. Isolate fish with mucus abnormalities in a 0.5% salt bath
2. Add a large dose of pH-raising chemical at once and bring it back to 7.5 within a few hours
3. Rainwater is natural water so there is no problem. Increase food to build the fish's strength
4. Drain the pond completely, fill it with tap water and reset it the same day

AnswerA. Remove the leaves and sludge, and change water stepwise with new water at the same temperature to bring pH back gently. Isolate fish with mucus abnormalities in a 0.5% salt bath

ExplanationProlonged rain brought in large amounts of acidic rainwater, and decomposing leaves lowered the pH further. A sudden pH drop damages the mucus, and at 15–20°C in fall bacterial infections such as hole-in-the-side disease are also likely. Remove the leaves and sludge to reduce the cause, and change water stepwise with new water at the same temperature to bring pH back gently. Isolate fish with cloudy mucus in a 0.5% salt bath to prevent infection, and consult a fish vet if they worsen. A large dose of pH-raising chemical causes another shock from the sudden swing. Rainwater is acidic and carries mud and pollutants, so natural does not mean safe. A full change loses the bacteria and chlorinated tap water damages the mucus.

IssuesThere was no leaf net for fall, and the habit of testing and acting on water quality after prolonged rain lagged. There was insufficient awareness that rainwater lowers pH and decomposing leaves worsen water quality further. Mucus abnormalities were nearly missed as a precursor to infection.

ImprovementStretch a leaf net over the pond in fall and remove leaves and sludge weekly. After prolonged rain or a typhoon, always test pH, ammonia and nitrite and change water stepwise if anything is off. Dig a drainage channel around the pond to reduce rainwater inflow. Keep an isolation container and salt on hand, and inspect the body surface daily during the spring and fall infection seasons.

In September the municipality issued an evacuation order for a large typhoon. Animals are not allowed at the shelter, and there are 20 large koi in the garden pond. A power outage is likely. You have one hour before leaving, two battery air pumps, netting and stored water. The family is saying that if the koi cannot come along, they should be released into the river.

Which is the most appropriate response?

1. Release the koi into the river to return them to nature and catch them again after the typhoon
2. Drain the pond, put all the koi in buckets, load them in the car and evacuate
3. Lower the water level and cover with netting, set up the battery air pumps, stop feeding, and evacuate with human safety first. On return, test and change the water
4. Call off the evacuation and keep watch by the pond overnight to protect the koi

AnswerC. Lower the water level and cover with netting, set up the battery air pumps, stop feeding, and evacuate with human safety first. On return, test and change the water

ExplanationWhen an evacuation order is issued, human safety comes first; leave the koi in the pond after preparing it, then evacuate. Before leaving, lower the water level to prevent escape, cover with netting against jumping out and predators, and set up the battery air pumps against an outage. Stop feeding to avoid fouling the water. On return, test and change the water, and consult a fish vet if anything is wrong. Releasing into the river is illegal release that spreads pathogens and affects the ecosystem. Loading 20 large koi in buckets in a car is impossible and causes oxygen depletion and spills. Calling off evacuation to spend the night at the pond endangers human life.

IssuesThere was no advance plan for the koi during evacuation, and the family had the illegal and dangerous idea of releasing them if they could not come along. Preparations for an outage or escape (water level, netting, air pumps) were left to the last minute.

ImprovementBefore typhoon season, write out the evacuation procedure (lower water level, netting, battery air pumps, stop feeding) and share it with the family. Tell every family member that release is illegal and spreads disease. Keep stored water and spare batteries on hand, and set the procedure for water testing and changes on returning home.

In June an acquaintance calls: their pond koi are dying one after another and they want you to take in the remaining healthy ones. Their pond is at 24°C, and the dead koi reportedly had white, rotting gills. They have not reported it and say the healthy fish will be fine. Your own pond holds 10 treasured koi.

Which is the most appropriate response?

1. Healthy fish are safe, so take them into your pond to help
2. Explain that KHV is strongly suspected, decline to move the fish, and urge them to report to the prefectural fisheries division or Livestock Hygiene Service Center
3. Before taking them in, put the acquaintance's koi in your pond for a few days to see how they do, then decide
4. Take the acquaintance's water and fish in a separate container at home and treat them yourself with medication

AnswerB. Explain that KHV is strongly suspected, decline to move the fish, and urge them to report to the prefectural fisheries division or Livestock Hygiene Service Center

ExplanationA run of sudden deaths at 18–28°C with gill rot is typical of koi herpesvirus disease (KHV). KHV is a notifiable disease; whoever finds it must report to the prefectural government, and moving fish from an infected pond is prohibited. Fish that look healthy often carry the virus, and taking them in could wipe out your own koi. Declining the move and urging a report is correct. The idea that healthy fish are safe is wrong, and putting them in your pond for a few days is the worst way to bring in infection. There is no effective treatment for KHV, and holding them in a separate container is itself an illegal movement.

IssuesWanting to help an acquaintance nearly led to downplaying KHV's infectiousness and the legal duties of a notifiable disease. Both parties held the mistaken belief that healthy-looking fish are safe, and knowledge of reporting duties and movement bans had not been shared.

ImprovementShare KHV symptoms and the duties on detection (report, no movement, no release) with family and fellow koi keepers. Accept fish from other ponds only with a health certificate and 3–4 weeks of quarantine, and never from a pond where sudden deaths are occurring. Keep the contact details of the prefectural fisheries division and Livestock Hygiene Service Center.

In May, with a small hangnail cut on a finger, you cleaned sludge from the pond and checked the koi's bodies bare-handed. Three days later the area around the cut is red, swollen and hot, and a line of lumps is spreading toward the wrist. There is no fever. One of the koi has had red spots on its body since last week.

Which is the most appropriate response?

1. Pond water will cool the inflammation, so soak the cut in the pond
2. It is a small cut, so just use an over-the-counter antiseptic and leave it until the swelling goes down
3. The koi may not be the cause, so ignore the koi's spots and treat the hand with over-the-counter medicine only
4. See a dermatologist or physician and tell them about contact with the koi pond water and fish. Use gloves from now on, and consult a fish vet about the koi's spots

AnswerD. See a dermatologist or physician and tell them about contact with the koi pond water and fish. Use gloves from now on, and consult a fish vet about the koi's spots

ExplanationPond water and fish surfaces carry bacteria such as nontuberculous mycobacteria and Aeromonas, and infection through a cut on the hand can cause red swelling and lumps spreading toward the wrist. These do not heal with ordinary wound care and need proper diagnosis and treatment, so see a dermatologist or physician and be sure to mention contact with pond water and fish. The koi's red spots may be a bacterial infection, so consult a fish vet separately. Soaking in pond water takes in more bacteria. Antiseptic alone lets the infection progress and the lumps spread. Ignoring the koi's symptoms leaves the source of infection in the pond.

IssuesPond work was done bare-handed with a cut, without knowing the risk of fish-borne bacterial infection. Handwashing after work and glove use were not habits, and awareness of zoonoses was low. The koi's skin spots and the owner's own symptoms were not connected.

ImprovementAlways wear waterproof gloves for pond work, and do not touch the water with cuts on the hands. Wash hands with soap after work. Children and the elderly should sanitize hands after pond work. Set a family rule: if a cut swells red after touching fish, see a doctor and report the fish contact. Consult a fish vet early about skin abnormalities on the koi.

In April, a koi bought at a sale is in quarantine in a separate container. In week two, the koi in the main pond have begun rubbing their bodies and have white spots. The quarantined koi shows nothing wrong. The same net and bucket were used for both containers, and you went back and forth without washing your hands. The pond temperature swings widely, 12°C in the morning and 18°C at midday.

Which is the most appropriate response?

1. Separate and disinfect nets, buckets and hands per container and manage both separately. Treat the pond fish with a 0.5% salt bath and stable temperature, and see a vet if it spreads
2. The quarantined koi is the cause, so dispose of it immediately
3. If both will get the same disease anyway, end the quarantine and add the new koi to the pond
4. White spots in the pond are a natural spring phenomenon, so leave them and just continue the quarantine

AnswerA. Separate and disinfect nets, buckets and hands per container and manage both separately. Treat the pond fish with a 0.5% salt bath and stable temperature, and see a vet if it spreads

ExplanationEven if the quarantined koi shows nothing, sharing nets and buckets and moving between containers without washing hands carries pathogens between them. To keep the quarantine meaningful, separate and disinfect tools per container, wash hands and manage them separately. White spot disease in the pond is often triggered by sudden temperature swings, so stabilize the temperature, reduce stress with a 0.5% salt bath, and consult a fish vet within a few days if it spreads. Disposing of the quarantined koi has no basis; shared tools are the likely cause. Ending quarantine and merging spreads infection. Dismissing white spot as natural lets it reach the gills and kill.

IssuesA quarantine container was set up, but hygiene to stop pathogens moving via nets, buckets and hands was lacking, defeating the purpose of quarantine. The owner did not know the link between spring temperature swings and white spot disease, and pond management was also inadequate.

ImprovementKeep color-coded nets, buckets and sponges dedicated to the quarantine container and never share them between containers. Wash hands and disinfect tools after working on each container, and continue quarantine for 3–4 weeks. Watch temperature stability in spring and fall and check daily for white spots and rubbing. Keep an isolation container and salt on hand.

In October, three of the koi in the pond died within a week. Their gills were not rotten, but they had red ulcers on the body like hole-in-the-side disease. The remaining five also have wounds. The family wants to drain all the pond water into the gutter, refill with tap water, and release the dead fish into the nearby river as a farewell.

Which is the most appropriate response?

1. As the family wishes, drain the pond into the gutter for a full change and release the dead fish into the river
2. Sink the dead fish in the pond as nutrition for the others and leave the water as is
3. Seal the dead fish in bags and dispose of them per municipal instructions, change the water stepwise without draining to the river. Isolate the remaining fish in a salt bath and consult a fish vet
4. Move all the fish to an acquaintance's pond and dry out your own pond

AnswerC. Seal the dead fish in bags and dispose of them per municipal instructions, change the water stepwise without draining to the river. Isolate the remaining fish in a salt bath and consult a fish vet

ExplanationFish that died of bacterial infections such as hole-in-the-side disease, and the pond water, contain pathogens. Seal the dead fish in bags and dispose of them under municipal rules, and do not drain the pond water into the gutter or river; improve water quality with stepwise changes. Isolate the remaining fish in a 0.5% salt bath to control infection and consult a fish vet on a treatment plan. Releasing dead fish into a river spreads pathogens into the wild and, beyond the problems under infectious-disease law, may itself be illegal. Sinking dead fish in the pond spreads decay and bacteria and worsens the other fish. Moving fish to an acquaintance's pond carries disease in, and is especially forbidden while KHV remains a possibility.

IssuesThe family did not know the correct way to handle dead fish and pathogen-laden pond water and thought of releasing them into the river as a farewell. No isolation or treatment was done before three fish died, and wounds had spread to the rest. Management in the fall hole-in-the-side season was inadequate.

ImprovementShare the family rules: seal dead fish in bags and dispose of them per municipal instructions, and never drain pond water outside. Isolate sick fish at the first case and consult a fish vet. Be thorough with fall leaf and sludge cleaning and water changes, and dedicate and disinfect pond tools. Do not exchange fish or water with acquaintances.

On a July evening, in a pond at 28°C, a 40 cm koi is lying on its side on the bottom, not moving. When approached, only its gill covers move slowly, but its eyes are moving. The other fish are fine. It was swimming normally this morning. You have an isolation tub, an air pump and salt on hand.

Which is the most appropriate first action?

1. A fish on its side cannot be saved, so bury it to prevent infecting the others
2. Move it gently with wet hands to the tub of pond water at the same temperature, aeration at maximum, 0.5% salt bath, dark and quiet, and consult a fish vet the same day
3. Hold the fish in the pond and shake it up and down underwater to bring it round
4. Lift the fish to the surface to let it breathe air and blow into its mouth

AnswerB. Move it gently with wet hands to the tub of pond water at the same temperature, aeration at maximum, 0.5% salt bath, dark and quiet, and consult a fish vet the same day

ExplanationEven on its side, if the gill covers and eyes are moving there is a chance of survival. Chest compressions and artificial respiration are not possible for fish, so instead optimize oxygen and water quality. Move it gently with wet hands to the tub, supply oxygen with pond water at the same temperature and aeration at maximum, reduce osmotic stress with a 0.5% salt bath, keep it dark and quiet and do not touch it until it swims on its own. It is a sudden collapse of unknown cause, so consult a fish vet the same day. Burying a fish with moving gills is far too early. Shaking underwater damages the organs and gills, and lifting it to breathe air or blowing into its mouth does not match how fish breathe, dries the gills and is fatal.

IssuesThe owner did not know fish life-saving measures and thought in terms of mammal-style artificial respiration or stimulation. The assumption that on its side means dead reflected a lack of knowledge about gill movement as a sign of life. The long gap between morning and evening observation delayed noticing the collapse.

ImprovementWrite out and post beside the pond the basics of fish life support: new water at the same temperature, maximum aeration, 0.5% salt bath, dark and quiet. Keep an isolation container, air pump and pre-measured salt on hand. Observe every fish's swimming, gill movement and fin posture twice a day, morning and evening. Confirm the fish vet's contact details and hours.

In September, in a pond at 24°C, a beginner one month into keeping koi wants to check their health. One koi's gill covers move fast at about 120 times a minute, while another's are calm at about 50 a minute. Both are eating. The beginner thinks the fast one is the lively one and worries about the slow one.

Which is the most appropriate judgment?

1. The fast one is lively, so isolate the slow koi in a salt bath
2. Both are normal, so keep feeding without recording anything
3. Gill movement varies by individual, so ignore it, increase food and watch
4. 50 per minute is normal; 120 is too fast. Observe the fast koi and test the water; if there is piping or one-sided stoppage, isolate and consider seeing a vet

AnswerD. 50 per minute is normal; 120 is too fast. Observe the fast koi and test the water; if there is piping or one-sided stoppage, isolate and consider seeing a vet

ExplanationAt around 24°C a koi's gill covers should move roughly 40–80 times a minute, so 120 is breathing too fast. It may signal oxygen shortage, gill parasites or bacterial infection, or breathing difficulty from ammonia or nitrite buildup. Observe the fast koi and test the water (ammonia, nitrite, pH). If there is piping, one gill cover stopping or darkened gills, isolate in a 0.5% salt bath and consult a fish vet. Isolating the slow one is backwards and causes needless stress. Writing both off as normal or individual variation misses early gill disease. Increasing food worsens water quality further.

IssuesAs a beginner, the owner did not know a koi's normal gill rate (about 40–80 per minute) and misread fast as lively. There was no habit of testing water and insufficient knowledge to suspect water quality or gill disease from abnormal breathing.

ImprovementPost the koi's normal values beside the pond (gill covers about 40–80 per minute, varying with temperature), and count and record at morning feeding. Test water weekly with a test kit, and the same day if any fish is breathing fast. Learn the signs of gill disease (one-sided stoppage, darkening, piping) and set criteria for isolating and seeing a vet when they appear.

In June a pond koi is bleeding from the base of a pectoral fin, trailing a thread of blood as it swims. Thirty minutes ago it thrashed when netted and hit the pond edge. The bleeding continues slowly, the pond water is slightly cloudy and has not been changed for two weeks. The water is 25°C, and you have an isolation tub and salt on hand.

Which is the most appropriate response?

1. Move it to an isolation container of clean new water, rest in a 0.5% salt bath, and consult a fish vet if bleeding does not stop or spreads
2. Press hard on the bleeding site with a finger and apply pressure for several minutes out of the water
3. Sprinkle human styptic powder on the wound before returning it to the pond
4. Bleeding will not stop underwater, so seal the wound with superglue

AnswerA. Move it to an isolation container of clean new water, rest in a 0.5% salt bath, and consult a fish vet if bleeding does not stop or spreads

ExplanationBleeding from a fish's fins or body usually stops naturally with rest in clean water. Cloudy pond water is high in bacteria and the risk of secondary infection is high, so move the fish to an isolation container of clean new water, reduce osmotic stress with a 0.5% salt bath and rest it in the dark and quiet. If bleeding does not stop, or the wound swells white or spreads red, consult a fish vet. Pressure out of the water dries the gills, damages the mucus and makes the fish thrash, worsening the bleeding. Styptic powder and glue do not work underwater and damage the mucus and wound. Change the pond water in parallel to limit bacterial growth.

IssuesNetting made the fish thrash and hit the pond edge, causing the injury. Water changes were neglected for two weeks, raising infection risk in cloudy water. The owner did not know that for fish, stopping bleeding means rest in clean water, not pressure out of the water.

ImprovementCatch fish by guiding them underwater into a plastic bag or tub without letting them thrash. Pad the pond edge and sharp stones. Keep to weekly one-third water changes and change water immediately if it clouds. Keep an isolation container, air pump and salt on hand, and post the bleeding procedure (clean water, salt bath, rest, criteria for seeing a vet).

At midday in August, a 70 cm koi with a swollen belly and raised scales needs to see a fish vet. The clinic is an hour away by car and the air temperature is 34°C. You have a large plastic bag, a foam box, a battery air pump and ice packs. You are unsure whether to fill the bag with pond water or tap water.

Which is the most appropriate transport method?

1. Put tap water in the bag and carry it in the cargo area in direct sun to keep it warm
2. Use no water; wrap it in a wet towel and carry it in a cooler
3. Put pond water in the bag, supply oxygen with the battery air pump, place it in the foam box with ice packs in the corners, keep it dark and carry it without jolting
4. Fill the bag completely with water and no air so the fish cannot move during the trip

AnswerC. Put pond water in the bag, supply oxygen with the battery air pump, place it in the foam box with ice packs in the corners, keep it dark and carry it without jolting

ExplanationFor transport use the pond water the fish is used to, supply oxygen with a battery air pump, put the bag in a foam box with ice packs in the corners (not touching the bag), keep it dark, quiet and insulated, and travel without jolting by the shortest route. Bring pond water, photos of the fish and records of temperature and water quality to the clinic. Tap water damages the mucus and gills with chlorine, and heating in direct sun lowers oxygen, which is fatal. Transport without water dries the gills and suffocates the fish within minutes. A bag full of water with no air lacks oxygen, and being unable to move is highly stressful.

IssuesThe owner did not know how to transport a large fish (pond water, oxygen, insulation, dark and quiet) and considered tap water or no water. The risks of oxygen shortage and rising temperature in midsummer heat were not understood. Information to bring to the clinic (water records, photos) was not prepared either.

ImprovementKeep a transport kit for large fish (large bag, foam box, battery air pump, ice packs) and write out the transport procedure. In summer travel in the cool of early morning or evening. Keep daily records of temperature, pH, ammonia and nitrite and bring them to the vet. Confirm the fish vet's contact details and hours in advance.

At 1 a.m. on an August night, getting up for the bathroom, you look at the pond and see 8 of the 15 koi sluggish near the surface with mouths open, and 2 lying on their sides. The air pump is running but the waterfall pump has stopped. The water is 30°C. The piping fish still have moving gills.

Which is the most appropriate first action?

1. It is late, so wait until morning and contact a fish vet when it is light
2. Immediately maximize aeration, restart the pump and change water with new water at the same temperature. Isolate the fish on their sides with aeration, and contact an after-hours clinic
3. Isolate only the two fish on their sides and leave the rest in the pond until morning
4. Move all the fish to indoor buckets, fill them with tap water and keep them until morning

AnswerB. Immediately maximize aeration, restart the pump and change water with new water at the same temperature. Isolate the fish on their sides with aeration, and contact an after-hours clinic

ExplanationSeveral fish piping and rolling over at once signals pond-wide oxygen depletion or a sudden water-quality change; on a hot night dissolved oxygen is at its lowest and many would die by morning if left. Immediately maximize aeration, restart the pump to restore flow, and change water with new water at the same temperature to improve oxygen and water quality. Isolate the fish on their sides with aeration and reduce stress with a 0.5% salt bath. If there is an after-hours clinic that treats fish, contact it for instructions. Waiting until morning is fatal, and isolating only the two does not fix the pond-wide oxygen shortage. Moving every fish to indoor buckets worsens oxygen shortage through overcrowding, and the chlorine and temperature difference of tap water damage the mucus.

IssuesThe stopped waterfall pump went unnoticed, and aeration was insufficient for the oxygen-depletion risk of a summer night. There was no criterion that several fish showing symptoms at once means a pond-wide environmental problem, and treating individuals or waiting until morning was considered.

ImprovementIn summer run multiple aeration systems for redundancy, and set up an alarm for pump stoppage or a nightly check. Keep a battery air pump and stored water on hand. Make it a rule that several fish in trouble at once means act now, and look up a fish vet that can be consulted at night. Consider a dissolved-oxygen meter.

In June, a koi that jumped out and lay on its side has spent two days in an isolation container with a 0.5% salt bath and aeration. This morning it swims upright on its own, fins open and showing interest in food. The abrasion on its body is covered by a thin white film with no redness. The pond is 23°C and the isolation container 22°C. The family says it is well now, so put it straight back in the pond.

Which is the most appropriate response?

1. It is well, so toss it straight from the isolation container into the pond and give a normal amount of food right away
2. To be safe, keep it in the isolation container another month, raising the salt to 1% until fully healed
3. Pour the whole salt bath into the pond, returning the koi with it
4. Dilute the salt over several days, match the temperature to the pond, then return it gently. Feed lightly for a few days afterward and watch the wound and its swimming

AnswerD. Dilute the salt over several days, match the temperature to the pond, then return it gently. Feed lightly for a few days afterward and watch the wound and its swimming

ExplanationSwimming on its own, fins open and interest in food after life-saving care are signs of recovery, but an abrupt change of environment can cause renewed shock. Dilute the salt gradually with water changes over several days, match the temperature to the pond (acclimate in a bag or tub) and return it gently. Feed lightly for a few days afterward, watch the wound and its swimming, and if redness or clouding appears, isolate again and consult a fish vet. Tossing it straight in is dangerous from the abrupt change in water quality and temperature and the impact. A month more in isolation at 1% salt is a heavy burden, and strong salt water weakens fish over the long term. Pouring the salt bath into the pond changes the whole pond's salinity and water quality and affects the other fish.

IssuesThe owner did not know how to end life-saving care (acclimation, gradual salt removal) and tended to judge that well means put it back now. There were no criteria for follow-up after isolation, and knowledge of salt bath concentration and duration was also insufficient.

ImprovementWrite out the procedure for returning from isolation (salt dilution over several days, temperature matching, gentle transfer, several days of observation) and share it with the family. Set recovery criteria (swims upright, fins open, eats, no redness at the wound). Observe the body surface and swimming daily for a week after return, and keep the container set up rather than putting it away so the fish can be isolated again immediately if it relapses.