Japanese freshwater fish case method: 50 questions

Late October, morning. Of 6 pale chub collected in summer and kept in a 45 cm tank, 2 have white dots like grains of salt scattered over their bodies and fins. One is rubbing itself against the driftwood. Nights have been cold for the past few days, and the unheated tank had dropped to 16°C in the morning.

What is the most appropriate first action?

1. Scrub the white dots off with a cotton swab, then return the fish to the tank
2. Immediately raise the water to 30°C in one go to kill the parasites
3. Change 1/3 of the water with same-temperature water and raise the temperature 1–2°C per day to about 26°C
4. Crush human antibiotics found online and add them to the tank

AnswerC. Change 1/3 of the water with same-temperature water and raise the temperature 1–2°C per day to about 26°C

ExplanationWhite dots plus rubbing are classic white spot disease (Ich), which tends to break out when a sudden fall temperature drop lowers the fish's immunity. Because the parasite stops multiplying at higher temperatures, the basic approach is to change 1/3 of the water with same-temperature water to reduce free-swimming parasites, then use a heater to raise the temperature gradually, 1–2°C per day, to about 26°C. Jumping straight to 30°C will kill Japanese freshwater fish accustomed to cool water by temperature shock. The dots cannot be scrubbed off; scrubbing damages the slime coat and opens the door to water mold. Human medication is of unknown type and dose and is dangerous. If it spreads over the whole body or does not improve, use fish medication in a separate container as directed and consult a vet who treats fish the same day.

IssuesThe thermometer was not checked at the change of season, so the sharp overnight drop went unnoticed. The assumption that Japanese freshwater fish can be kept unheated meant there was no preparation for sudden changes.

ImprovementEven in unheated setups, check the thermometer every morning in fall and winter, and if the temperature changes 2°C or more in a day, stabilize it gently with a heater. Inspect every fish's body and fins once a day and do not miss rubbing behavior.

A January night. A 15 cm crucian carp in a 60 cm tank has fluffy white cotton-like growth on its snout and belly. A week ago, several scales were knocked off when it was moved with a net. The room is unheated, the water is around 10°C, and the fish is barely eating.

What is the most appropriate first action?

1. Peel the cotton off with tweezers and put the fish straight back in the tank
2. Move to an isolation container with a 0.5% salt bath and raise the temperature 1–2°C per day until stable
3. Fish normally do not eat in winter, so do nothing and wait until spring
4. Add diluted household disinfectant to the whole tank to kill the mold

AnswerB. Move to an isolation container with a 0.5% salt bath and raise the temperature 1–2°C per day until stable

ExplanationThis is a classic case of water mold taking hold on net abrasions in cold water. Cotton wool disease spreads from the wound, and once the snout is covered the fish cannot eat and wastes away. Isolate it in a separate container, protect the skin with a 0.5% salt bath, and raise the temperature 1–2°C per day to about 18–20°C, which slows the fungal growth and helps healing. Forcibly peeling the cotton enlarges the wound and causes recurrence. Left until spring, the growth covers the whole body and the fish dies. Household disinfectant is deadly to fish and to filter bacteria and must never be added. If it spreads within days or the fish does not eat, use fish medication as directed in a separate container or consult a vet who treats fish the same day.

IssuesThe fish was handled by rolling it on a net, knocking off scales. The owner did not know that wounds in cold water invite water mold, left the wound untreated, and did not observe closely enough for a week.

ImprovementMove fish in a cup or bucket with water, not a net, and do not touch them with dry hands. If a fish is injured, isolate it immediately and protect it with a salt bath. Even in winter, check the snout and skin several times a week to catch white growth early.

Early August, evening. A 45 cm tank holds 12 bitterlings and 4 pale chub. Several fish have cloudy white tail-fin tips that look melted and shortened. One has a white, ulcerated snout. The water is 29°C and has not been changed for 2 weeks. Foam lingers on the surface without disappearing.

What is the most appropriate first action?

1. Trim the rotted fins with scissors to encourage regrowth
2. Put ice directly into the tank to lower the temperature
3. Isolate the affected fish, change 1/3 of the main tank with same-temperature water, and relieve the overcrowding
4. Increase feeding to build strength and wait for natural healing

AnswerC. Isolate the affected fish, change 1/3 of the main tank with same-temperature water, and relieve the overcrowding

ExplanationCloudy, rotting fin tips and an ulcerated snout are columnaris (fin rot / mouth rot), which spreads rapidly in warm, overcrowded, dirty water. The priority is to isolate the affected fish and treat them with a 0.5% salt bath and daily water changes, change 1/3 of the main tank with same-temperature water, and split the fish into another tank to reduce density. Cutting fins lets infection enter through the wounds. Ice added directly causes shock from the sudden temperature difference. More food means more leftovers and worse water. Keep the water below 28°C with a cooling fan; if the snout ulcer progresses or the fish stop eating, use fish medication as directed in a separate container and consult a vet who treats fish the same day.

Issues16 fish in a 45 cm tank is clearly overcrowded, and water changes were skipped for 2 weeks in summer. The persistent surface foam was a sign of excess organic matter but was overlooked. There was little understanding that bacteria multiply readily in warm water.

ImprovementLimit a 45 cm tank to 5–8 small fish and add tanks as numbers grow. In summer, never skip the weekly 1/3 water change and keep the water below 28°C with a cooling fan. Check the surface for foam, odor and cloudiness daily and catch cloudy fin tips early.

Mid-April, morning. Of 3 crucian carp overwintered in an outdoor water-lily bowl, one has 2 red, gouged-out holes on its flank where scales have fallen off. Days have been warm and mornings cold, with the water swinging from 13°C in the morning to 20°C at midday. The fish sits still on the bottom and does not eat.

What is the most appropriate first action?

1. Apply an over-the-counter wound ointment directly to the holes and return it to the outdoor bowl
2. Move it to an indoor isolation container, keep the water stable at 22–25°C, with daily 1/3 water changes and a 0.5% salt bath
3. Spring parasites are the cause, so add a dewormer to the bowl
4. It will heal on its own as it warms up, so increase feeding and watch

AnswerB. Move it to an indoor isolation container, keep the water stable at 22–25°C, with daily 1/3 water changes and a 0.5% salt bath

ExplanationRed, gouged-out holes on the flanks of crucian carp and bitterlings are hole-in-the-side disease (atypical Aeromonas infection), which breaks out in spring and fall when the water hovers unstably around 15–20°C. The bacteria are active at cooler temperatures, so the basic treatment is an indoor isolation container with the water held stable at 22–25°C, daily 1/3 water changes and a 0.5% salt bath to protect the skin. Human wound ointment washes off in water and its ingredients are harmful to fish. It is not a parasite, so dewormers have no effect. Left alone, the holes deepen into the muscle and the fish dies. If the holes spread or the fish does not eat for 2–3 days, consult a vet who treats fish the same day.

IssuesIn the outdoor spring setup the water swung 7°C between morning and midday, putting heavy stress on the fish. The owner did not know that hole-in-the-side disease strikes at seasonal transitions and was slow to notice the loss of appetite.

ImprovementIn spring and fall, place the bowl in shade or against a wall and use a reed screen or lid to reduce the day-night temperature swing. Record the water temperature every morning and move the fish indoors when the daily swing exceeds 5°C. Isolate immediately on seeing raised scales or redness on the flanks.

A July weekend, 5 dark chub were collected from a nearby river. On close inspection the next day, 2 have white thread-like things about 1 cm long sticking into their flanks, with red swelling at the base. The fish occasionally thrash and rub against stones. They have already been put in the main tank with the pale chub.

What is the most appropriate first action?

1. Pull the worms out from the base with tweezers, isolate all the collected fish in a separate container, and give a salt bath
2. The worms will fall off eventually, so keep them in the main tank as is
3. Add a small amount of household insecticide to the main tank to kill the worms
4. Take the fish out and wipe their bodies with a dry towel

AnswerA. Pull the worms out from the base with tweezers, isolate all the collected fish in a separate container, and give a salt bath

ExplanationThread-like worms embedded in the body are anchor worms, a parasite common on wild-caught fish. Slowly pull the adults out from the base with tweezers and protect the wounds with a 0.5% salt bath. Eggs and larvae are very likely already in the water, so isolate all the collected fish in a separate container, change the main tank's water, and watch for recurrence for 2–3 weeks. Left alone, the attachment sites become bacterially infected and larvae spread through the whole tank. Household insecticide is a deadly poison that kills fish instantly. A dry towel strips the slime coat and leads to water mold. If many are attached or they keep recurring, use antiparasitic fish medication at the specified dose in a separate container, or consult a fish shop or vet the same day.

IssuesThe collected fish were put straight into the main tank without observation or quarantine. The owner lacked the knowledge that wild fish often carry anchor worms, fish lice and Ich.

ImprovementAlways treat collected fish in a separate container for 1–2 weeks, inspecting the skin, fins and gills daily before adding them. Never put river water or the collection water into the main tank. Keep tweezers and an isolation container on hand so parasites can be dealt with immediately.

Mid-August, 6 a.m. All 10 pale chub and dark chub in a 60 cm tank are gathered near the surface, gasping. Their gills are moving fast. Last night was a tropical night, the thermometer reads 30°C, and the aeration was switched off the evening before because it was "too noisy." Yesterday's leftover food is lying on the bottom.

What is the most appropriate first action?

1. Move all the fish to a bucket and wait for them to recover
2. Restart the aeration, change 1/3 to half the water with same-temperature water, and siphon out the leftover food
3. Feed them to see if they perk up
4. Float ice directly in the tank to lower the temperature quickly

AnswerB. Restart the aeration, change 1/3 to half the water with same-temperature water, and siphon out the leftover food

ExplanationGasping at the surface with rapid gill movement is classic oxygen depletion. Water at 30°C holds little dissolved oxygen, and stream-dwelling pale chub and dark chub have especially high oxygen needs, so stopping the aeration plus rotting leftover food caused a sudden crash. Restart the aeration immediately, change 1/3 to half the water with same-temperature water to reduce organic matter, and siphon out the leftovers. A bucket has even less oxygen and is dangerous. Stop feeding, since digestion uses oxygen and fouls the water. Ice added directly causes localized rapid chilling and shock, so float frozen plastic bottles and lower the temperature gradually. If gasping continues after the water change or any fish lies on its side, it is an emergency; consult a vet who treats fish right away.

IssuesThe summer aeration was switched off over noise, without understanding how much oxygen stream fish need. Leftover food plus 30°C water combined to cause a mass oxygen crash in the morning.

ImprovementNever switch off aeration in summer; switch to a quiet pump instead. Keep the water below 28°C with a cooling fan or air conditioning and siphon out leftover food daily. Keep frozen plastic bottles ready for emergency cooling when the water exceeds 30°C.

A May night, day 3 after setting up a new 45 cm tank. 4 collected gobies and 6 bitterlings were added. The gobies are sitting motionless on the bottom with rapid gill movement, and the bitterlings are gasping at the surface. The water is clear but smells slightly. Test strips show ammonia and nitrite.

What is the most appropriate first action?

1. Do nothing and wait a few days for the filter to start working
2. Replace all the water with fresh tap water and reset
3. Change half the water with same-temperature dechlorinated water, stop feeding, and keep changing water while testing daily
4. Feed more to grow the bacteria

AnswerC. Change half the water with same-temperature dechlorinated water, stop feeding, and keep changing water while testing daily

ExplanationRight after setup, the bacteria that break down ammonia and nitrite have not yet developed, and adding fish all at once causes poisoning. Rapid gill movement and gasping are signs of poisoning. The correct response is to change half the water with same-temperature dechlorinated water to lower the concentration, stop feeding to limit organic waste, and repeat water changes while testing daily. Waiting means death within days. A full change with undechlorinated tap water wipes out the fish with chlorine and kills what little bacteria had grown. More food means more ammonia. If any fish lies on its side despite water changes, consult a vet who treats fish right away.

Issues10 fish were introduced at once before the filter bacteria had developed. There was no habit of testing water during cycling, and the odor was noticed late. The owner assumed wild-caught fish are hardy.

ImprovementRun the filter for 2–3 weeks before adding fish to a new tank, and start with 2–3 fish. For the first month, test ammonia and nitrite every 2–3 days and change water immediately if detected. Keep test strips on hand.

A June night. A 20 cm crucian carp kept for 3 years in a 60 cm tank has a swollen belly, and seen from the side its scales stand out like a pinecone. Its eyes are slightly bulging. Because it eats well it has been fed 3 times a day, and the water is changed about once a month. Black grime has built up on the bottom.

What is the most appropriate first action?

1. Press the belly with your fingers to squeeze out what is inside
2. Start a 0.5% salt bath and daily water changes in an isolation container, and consult a vet who treats fish the same day
3. Keep feeding as before to build strength
4. Raised scales are a growth-phase change, so leave it

AnswerB. Start a 0.5% salt bath and daily water changes in an isolation container, and consult a vet who treats fish the same day

ExplanationRaised scales (dropsy), a swollen belly and bulging eyes are classic Aeromonas infection, triggered by dirty water and overfeeding. As it progresses, fluid builds up in the abdomen and survival becomes difficult, so start a 0.5% salt bath and daily water changes in an isolation container and consult a vet who treats fish the same day for a treatment plan. Pressing the belly damages internal organs and spreads fluid and bacteria inside the body. Continued feeding burdens the digestive tract and fouls the water. It is not a growth change; left alone, the fish dies. The main tank also needs the grime siphoned out and a review of water-change frequency.

IssuesFeeding 3 times a day with monthly water changes had chronically degraded the water. The owner did not recognize the black grime on the bottom as a breeding ground for bacteria, and was complacent because crucian carp are hardy.

ImprovementFeed 1–2 times a day, only what is eaten in 2 minutes, and change 1/3 of the water weekly while siphoning the bottom. Once a month, check from the side for raised scales and a swollen belly. Find a vet or specialist shop that treats fish in advance.

A September afternoon. Of 8 bitterlings collected last month, 3 have translucent disc-shaped things about 3 mm across clinging to their skin. On close inspection, they move slowly. The fish are swimming restlessly and slamming their bodies against the glass.

What is the most appropriate first action?

1. Take the fish out, gently remove the disc-shaped parasites with tweezers, isolate them, and change the main tank's water too
2. It is dirt on the skin, so blast it off with a strong current
3. The parasites will die in the cooler fall water, so just watch
4. Add a large amount of table salt to the tank to knock the parasites off at once

AnswerA. Take the fish out, gently remove the disc-shaped parasites with tweezers, isolate them, and change the main tank's water too

ExplanationMoving disc-shaped parasites are fish lice (Argulus), often brought in by wild-caught fish. The irritation makes fish thrash and slam into glass and stones, causing injuries. Gently remove the adults with tweezers, isolate the fish, and protect the wounds with a 0.5% salt bath. Eggs are laid on stones and glass in the tank, so change the main tank's water and observe for 2–3 weeks. A current will not remove them and only exhausts the fish. They overwinter even in cold water, so waiting invites recurrence. A large dose of salt kills the fish itself through sudden osmotic shock. If many are attached or recurrence continues, use antiparasitic fish medication at the specified dose in a separate container, or consult a fish shop or vet the same day.

IssuesPost-collection quarantine was skipped a month ago, allowing parasites in. Restless swimming was misread as being "lively," and close inspection of the skin was neglected.

ImprovementAlways quarantine collected fish in a separate container for 2 weeks and inspect the skin daily in good light. Remember that slamming and thrashing are signs of parasites, and check with a magnifying glass. Keep tweezers and an isolation container on hand.

Late January, evening. 3 gobies in an unheated indoor tank have barely responded to frozen bloodworms for about a week and sit still under the stones. The water is 9°C. No white spots, cotton or parasites are visible; gill movement is slow but regular, and they dart under the stones when approached.

What is the most appropriate first action?

1. Decide they are sick and add fish medication to the tank immediately
2. Add plenty of food every day until they eat, and leave it
3. Use a heater to raise the water to 25°C overnight to activate them
4. Judge it as normal reduced activity from the cold, cut feeding to 2–3 times a week, and remove leftovers

AnswerD. Judge it as normal reduced activity from the cold, cut feeding to 2–3 times a week, and remove leftovers

ExplanationGobies are cold-blooded; below 10°C their metabolism drops, and it is normal for them to sit still and eat very little. With no skin abnormalities, regular gill movement and a flight response, this is winter slowdown, not disease. Feed very small amounts 2–3 times a week and always remove uneaten food. Medicating a healthy fish only stresses it. Adding plenty of food daily leaves rotting leftovers, which break down slowly in cold water and degrade water quality. Raising the water to 25°C overnight is a sudden temperature change with a risk of fatal shock. If they still do not eat when the water warms in spring, lose weight, or lie on their side, suspect disease and see a vet.

IssuesUnaware of the winter physiology of Japanese freshwater fish, the owner nearly mistook reduced appetite for disease. Observation itself was done properly, but seasonal adjustment of feeding was unknown.

ImprovementBelow 15°C, cut feeding to 2–3 times a week; below 10°C, cut back further. In winter, use three points as the observation standard: skin, gill movement and flight response. Organize season-by-season knowledge for judging what is abnormal.

A November night. White spot disease in 6 pale chub was noticed a week ago; the water was raised to 25°C and changed, but the spots have spread over every fish's whole body and even the gills. Two are working their gills hard and are about to tip over near the surface. No medication has been used, and the vet who treats fish is an hour away by car.

What is the most appropriate next action?

1. Raise the water further to 32°C to kill the parasites for certain
2. Use fish medication in a separate container as directed, and phone the vet for advice tonight
3. The temperature increase is working, so continue the same treatment for another week
4. Return all the fish to the river and let nature take over

AnswerB. Use fish medication in a separate container as directed, and phone the vet for advice tonight

ExplanationWhite spot that does not improve with higher temperature and water changes needs a medicated bath, and with the gills infested and fish about to tip over, there is no time to spare. Use white-spot fish medication in a separate container as directed and phone the vet tonight for instructions. 32°C exceeds the limit for Japanese freshwater fish, and the drop in dissolved oxygen will suffocate fish with weakened gills. Another week of the same treatment will kill the 2 severely affected fish. Releasing collected fish introduces pathogens into wild populations and causes genetic contamination and must never be done. During treatment, increase aeration to ensure plenty of oxygen.

IssuesThe initial response was correct, but with no benchmark for judging effectiveness, it dragged on for a week. No fish medication was stocked and the vet's contact details for emergencies had not been looked up in advance.

ImprovementDecide in advance to switch to a medicated bath if white spot shows no improvement in 3–4 days. Keep fish medication, an isolation container and an air pump on hand. Post the contact details and hours of a vet and specialist shop that treat fish near the tank.

A July morning. 2 of 5 dark chub in a 60 cm tank are breathing with only one side, gill covers held wide open, and the inside of the gills looks whitish. They hover near the surface and are sluggish. The water is 27°C and was last changed 2 weeks ago. Last week, fish collected from a different river were added without quarantine.

What is the most appropriate first action?

1. Open the gill covers with your fingers and rinse out the debris inside
2. Lowering the temperature alone will cure it, so fit a cooling fan and watch
3. Isolate the affected fish in a 0.5% salt bath, change the main tank's water and boost aeration, and consult a vet the same day
4. Mix a supplement into the food to restore their strength

AnswerC. Isolate the affected fish in a 0.5% salt bath, change the main tank's water and boost aeration, and consult a vet the same day

ExplanationOne-sided breathing, flared gill covers and whitened gills are classic gill disease (bacterial or parasitic), which worsens rapidly as respiratory function declines. Isolate the affected fish in a 0.5% salt bath to ease the strain on the gills, change the main tank's water and boost aeration to secure oxygen, and consult a vet or fish shop the same day to identify the cause. Gills are extremely delicate; touching them with fingers causes fatal damage. Lowering the temperature alone does not remove the cause. Supplements do nothing for impaired gills. Adding new fish without quarantine very likely brought in parasites or bacteria.

IssuesFish collected from another river were added without quarantine. Water changes were skipped for 2 weeks, which combined with warm water to favor bacterial growth. There was no habit of checking the gills, so one-sided breathing was noticed late.

ImprovementAlways quarantine new fish for 2 weeks and check gill color and breathing before adding them. In summer, keep up weekly water changes and boost aeration. Get into the habit of looking head-on at how the gill covers open and whether the two sides differ, to catch one-sided breathing early.

A June morning at 7, a dark chub is found lying on the floor in front of the tank. The lid was left off after last night's water change. Its body is half dry and dusty and it looks motionless, but on close inspection the gills are moving slightly. The tank water is 23°C.

What is the most appropriate first action?

1. Wipe off the dust with a dry tissue, then return it to the tank
2. Pick it up gently with wet hands, return it to same-temperature tank water, support it near the aeration, and watch without touching
3. Decide it is dead and dispose of it
4. Rinse the body under tap water, then return it to the tank

AnswerB. Pick it up gently with wet hands, return it to same-temperature tank water, support it near the aeration, and watch without touching

ExplanationAs long as the gills are moving, recovery is possible. Pick it up gently with wet hands and return it to same-temperature tank water. If it cannot swim on its own, support it near the aeration with its mouth facing the current so water flows over the gills, and keep it dark and quiet without touching until it moves. A dry tissue strips the slime coat and leads to water mold and bacterial infection. Disposing of a fish whose gills are moving throws away the chance to save it. Chlorinated tap water damages the gills and slime coat. After recovery, add a slime-coat protectant at the specified dose and watch for white cotton on the skin for several days. If the gills do not move for hours after return, or it starts moving but stays on its side, consult a vet who treats fish.

IssuesAwareness that dark chub leap high out of the water was weak, so the lid was forgotten after the water change. There was no routine for confirming the lid was on, and the fish was not found until the next morning.

ImprovementAlways use a gap-free lid with a weight on dark chub and pale chub tanks, and close cord gaps too. After water changes, say "lid check" out loud to make it a habit. Check the floor after any work with the lid open, and keep a slime-coat protectant on hand.

During a July river outing, a child caught a pale chub with a hand net, rolled it around on the net to look at it, then grabbed it with dry hands and put it in a bucket. Back home, several scales on its flank are missing and the bare patch is red. The fish is swimming but holding its fins slightly clamped.

What is the most appropriate first action?

1. Apply human antiseptic to the wound with a cotton swab
2. Put it in the main tank with the other fish right away to settle it in
3. Use a 0.5% salt bath and slime-coat protectant in a separate container, and watch for several days for cotton-like growth
4. The wound will heal by itself, so put it in the main tank with no special care and feed generously

AnswerC. Use a 0.5% salt bath and slime-coat protectant in a separate container, and watch for several days for cotton-like growth

ExplanationRolling a fish on a net and grabbing it with dry hands strips scales and the slime coat, opening an entry point for water mold and bacteria. The right response is a 0.5% salt bath and slime-coat protectant in a separate container to protect the wound, watching for white cotton-like growth for several days. Human antiseptic is toxic to fish and damages the slime coat further. In the main tank it would be pecked by other fish, the wound would spread, and pathogens would be introduced. Generous feeding does not heal the wound and fouls the water. If cotton appears, treat for water mold; if the wound becomes red and gouged, consider fish medication or a vet visit. Collected fish should in any case be quarantined for 2 weeks, injured or not.

IssuesThe child had not been taught how to handle fish and was allowed to roll it on the net and grab it with dry hands, damaging the slime coat. Knowledge of post-collection quarantine was also lacking.

ImprovementWhen collecting, scoop fish with a bucket or cup of water and do not take them out into the air. If touching is necessary, use wet hands and be brief. Before collecting, teach children that fish are protected by a slime coat. Prepare an isolation container, salt and slime-coat protectant before coming home.

An August afternoon at 2 p.m. 8 bitterlings and pale chub collected from a river were put in a black bucket on the car's back seat for the hour-long drive home. On arrival the bucket water feels warm to the touch, half the fish are on their sides, and the rest are gasping at the surface. The home tank is 25°C.

What is the most appropriate first action?

1. Put them all straight into the 25°C tank to cool them
2. Add aeration to the bucket and mix in cool water little by little, lowering the temperature over about an hour
3. Put ice directly in the bucket to cool it fast
4. Decide the fish on their sides are dead and put only the rest in the tank

AnswerB. Add aeration to the bucket and mix in cool water little by little, lowering the temperature over about an hour

ExplanationThis is heat shock from high water temperature and oxygen depletion in a sun-baked car. Even fish on their sides can recover if the gills are moving, but a sudden temperature change would be a second blow. First add aeration to supply oxygen, then mix in cool water little by little, lowering the temperature to match the tank over about an hour, and keep it dark and quiet. Dropping them from warm water straight into the 25°C tank causes fatal shock from the temperature gap and introduces pathogens into the main tank. Ice added directly chills locally and is dangerous. Fish on their sides can be saved if their gills are moving, so do not give up on them. After recovery, observe in an isolation container for 2 weeks.

IssuesCollecting on a midsummer afternoon and leaving a black bucket in the car combined heat and oxygen depletion. There was no cooling or aeration gear, and the length of the drive was not considered.

ImprovementCollect in the morning in summer and keep the water below 25°C with a cooler box and ice packs. Always bring a battery-powered air pump and keep the number of fish small. Avoid direct sun during transport, and after getting home match the temperature before moving the fish to an isolation container.

A June evening. Of 5 pale chub in a 45 cm tank, one male whose red and blue breeding colors have become vivid is fiercely chasing the others. The small fish being chased has a torn tail fin and is stuck motionless in a corner. The tank has no hideouts, no driftwood and no stones.

What is the most appropriate first action?

1. Chase the male with a net, catch it, and put it in a separate bucket as punishment
2. Move the chased fish to a separate container of same-temperature tank water, and build hideouts in the main tank with driftwood and plants
3. It is natural breeding-season behavior, so do nothing and watch
4. Take the male out of the tank and return it to the river

AnswerB. Move the chased fish to a separate container of same-temperature tank water, and build hideouts in the main tank with driftwood and plants

ExplanationIn the breeding season, a male pale chub in breeding colors becomes strongly territorial and chases others hard. A fish with a torn tail fin stuck in a corner risks exhaustion and infection, so first move it to a separate container of same-temperature tank water to recover. In the main tank, build hideouts with driftwood, plants and stones that block line of sight to ease the chasing. Chasing the male with a net stresses and injures every fish. Left alone, the chased fish dies. Returning collected fish to the river causes genetic contamination and introduces pathogens and is strictly forbidden. Torn fins regrow in clean water, but if cloudiness or rot appears, switch to columnaris treatment.

IssuesPale chub were kept in a tank with no hideouts, with no expectation of male aggression in the breeding season. The chased fish's exhaustion was dismissed as merely "low energy" and isolation was delayed.

ImprovementCreate several refuges with driftwood, plants and stones in pale chub and dark chub tanks. From spring to summer, check daily for male breeding colors and chasing, and split them into another tank if it gets too intense. Keep a small isolation container and air pump on hand.

A May night. A 30 cm tank holds 4 gobies but only one flat stone. The largest has taken over the space under the stone and gapes and bites at any fish that comes near. The smallest has a torn dorsal fin and white bite marks on its flank, and has stopped moving in a corner near the surface.

What is the most appropriate first action?

1. Isolate the injured fish, and place at least one flat stone or pipe per fish in the main tank
2. Chase the largest fish with a net to stop its threatening
3. Gobies are naturally schooling fish, so they will get along eventually
4. Lower the water temperature to slow them down and reduce fighting

AnswerA. Isolate the injured fish, and place at least one flat stone or pipe per fish in the main tank

ExplanationGobies hold territories under stones and fight fiercely when hideouts are short. Isolate the injured fish in same-temperature tank water, protect the wounds with a 0.5% salt bath, and let it recover. In the main tank, place at least one flat stone or pipe per fish, positioned so they block each other's line of sight, so every fish can hold a territory. Chasing with a net does not change territorial behavior and only adds stress and injury. Gobies are not schooling fish and will not get along. Lowering the temperature only slows metabolism, solves nothing, and a sudden change is itself dangerous. 4 fish in a 30 cm tank is overcrowded, so consider a larger tank or fewer fish. If bite wounds turn cloudy or grow cotton, switch to disease treatment.

IssuesUnaware of goby territoriality, the owner kept 4 in a small tank with a single hideout. Threatening and biting were not recognized as an environmental problem, and the weak fish's injuries were noticed late.

ImprovementProvide at least one stone or pipe per goby and reduce density with a tank of 45 cm or more. Do not house fish of very different sizes together. Check daily for bite marks and torn fins, and separate any fish that keeps threatening others.

A September night. A small bitterling is stuck belly-first to the intake of the external filter, moving only its fins. The intake has no sponge and the flow is set high. The fish is about 3 cm, smaller than the others. It was collected only 2 weeks ago and is still thin.

What is the most appropriate first action?

1. Grab the fish and pull it off by force
2. Switch off the filter to stop the suction; if it does not come free on its own, gently detach it with wet hands and isolate it
3. Leave the filter running and wait for the fish to free itself
4. Turn the flow to maximum to blow the fish off

AnswerB. Switch off the filter to stop the suction; if it does not come free on its own, gently detach it with wet hands and isolate it

ExplanationA fish stuck to the intake cannot get free until the suction stops, and its belly and gills are being damaged. First switch off the filter to stop the suction; if it does not come free on its own, gently detach it with wet hands. Check for damage and let it recover in an isolation container of same-temperature tank water with a 0.5% salt bath and slime-coat protectant. Pulling by force while the suction is on tears the belly and scales. Left alone, it weakens and dies. Maximum flow only increases the suction. To prevent recurrence, fit a sponge on the intake and reduce the flow for small species. If there is a red gouge or bleeding on the belly, or it cannot swim after several hours, consult a vet who treats fish.

IssuesNo sponge was fitted on the intake, and a strong flow was used in a tank shared with a small bitterling. Settings meant for stream fish were applied unchanged to a weak small species, and the risk of nighttime suction was not foreseen.

ImprovementAlways fit a strainer sponge on the intake and reduce the flow in tanks with small species. In a mixed tank, adjust equipment to suit the smallest fish. Check around the filter daily to prevent nighttime suction accidents.

A November night. The room light was switched on suddenly after lights-out, and a dark chub shot into the glass at full speed. Its snout is red and crushed, several scales have come off, and right afterward it is on its side and wobbling. The gills are moving, but it swims at a tilt.

What is the most appropriate first action?

1. Dim the lights, keep it dark and quiet, do not touch it until it moves, and keep the aeration on low
2. Scoop it out with a net right away, move it to a separate container, and check the injury
3. Apply human ointment to the snout wound
4. Hold it by hand and keep it swimming until it recovers

AnswerA. Dim the lights, keep it dark and quiet, do not touch it until it moves, and keep the aeration on low

ExplanationRight after a collision, the fish is in something like a concussion, and the stimulus of touching or moving it triggers more panic and a second collision. Since the gills are moving, first dim the lights, keep it dark and quiet, and wait without touching until it can move. Keep the aeration on low to ensure oxygen. Netting and moving it makes it thrash and crash again, worsening the injuries. Human ointment washes off in water and leaves harmful residue. Holding it continuously exhausts it with stress. Once it settles after a few hours, inspect the snout wound; if cotton or cloudiness appears, isolate it and start a salt bath. If it is still on its side or swimming tilted after several hours, or cannot close its mouth, consult a vet who treats fish.

IssuesThe owner did not know that switching the lights on suddenly after lights-out startles dark chub into crashing. Awareness that dark chub and pale chub bolt at high speed when startled was lacking.

ImprovementTurn lights on and off in stages using indirect room lighting, avoiding sudden changes between light and dark. Build hideouts with plants and driftwood so startled fish have somewhere to retreat. Do nighttime work by a small light.

A March afternoon. A 20 cm crucian carp in a 60 cm tank has one eye that is cloudy white and slightly bulging. Yesterday, a sharp piece of driftwood was added during a layout change and the fish thrashed afterward. The other eye is normal and the fish is eating. A water test shows no nitrite.

What is the most appropriate first action?

1. Wipe the cloudiness off the eye with a cotton swab
2. Remove the sharp driftwood, give a 0.5% salt bath in an isolation container, and see a vet if it worsens
3. Only one eye is affected, so leave it
4. Add a strong fish medication to the whole tank at double the specified dose as a precaution

AnswerB. Remove the sharp driftwood, give a 0.5% salt bath in an isolation container, and see a vet if it worsens

ExplanationCloudiness and bulging in one eye only, following thrashing after sharp driftwood was added, points most strongly to traumatic corneal injury. Remove the driftwood that caused it and give a 0.5% salt bath in an isolation container to prevent infection while it recovers. The eye heals poorly, and wiping it with a cotton swab injures the cornea further. Even in one eye, infection can lead to blindness or loss of the eye if left alone. Overdosing fish medication poisons the fish itself. Cloudiness and bulging in both eyes would suggest poor water quality or Aeromonas, but here the water is fine. If the cloudiness spreads, the area around the eye becomes red and swollen, or the fish stops eating, consult a vet who treats fish the same day.

IssuesSharp driftwood was added without a safety check, with no thought that a large crucian carp might thrash and collide. The fish's behavior was not observed right after the layout change, so the eye problem was not noticed until the next day.

ImprovementUse only driftwood and stones with the edges removed, so fish are not injured even if they collide. Move fish to a temporary container during layout changes and observe their behavior for an hour after returning them. Make a daily habit of comparing the two eyes.

An April night. A week after switching the substrate to new, large, sharp-edged gravel, 2 gobies have red abrasions on the belly and the sucker-like pelvic fins, with scales rubbed off and turned whitish. Gobies move by crawling along the bottom and scrape their bellies on the gravel when burrowing under stones.

What is the most appropriate first action?

1. Rub salt directly into the red areas to disinfect them
2. Isolate the gobies in a salt bath and switch the substrate back to rounded, fine river sand
3. Leave the gravel until the gobies get used to it
4. Remove all the substrate, leave the glass bottom bare, and keep them that way

AnswerB. Isolate the gobies in a salt bath and switch the substrate back to rounded, fine river sand

ExplanationGobies are bottom dwellers whose pelvic fins have evolved into a sucker; they move on their bellies and burrow under stones. Large, sharp-edged gravel injures the belly and sucker, and bacteria or water mold enter where scales are lost. Isolate the gobies, protect the wounds with a 0.5% salt bath, and switch the substrate back to rounded, fine river sand. Rubbing salt in directly destroys tissue through osmotic damage. Leaving the gravel means the wounds never heal and get worse. A bare tank works for treatment, but for permanent keeping gobies cannot settle, and even with stone hideouts they still scrape their bellies. If the wounds become red and gouged, grow cotton, or the fish can no longer cling with its pelvic fins, consider disease treatment and a vet visit.

IssuesThe gravel was chosen for looks, without understanding the goby's bottom-dwelling ecology and belly-crawling body. The belly side was not inspected during the week after the substrate change.

ImprovementUse rounded, fine river sand or smooth gravel in tanks for bottom dwellers. Before changing substrate or layout, check that it suits the species. Inspect bottom dwellers from the side and below, not just from above.

An October morning. 15 bitterlings collected the previous day were left overnight in a small bucket. In the morning they have been colliding with each other; most have torn tail fins and skin injuries. The water is cloudy white, and 2 are on their sides with faintly moving gills. The main tank already holds resident bitterlings.

What is the most appropriate first action?

1. Move them all into the main tank right away to recover in a bigger space
2. Dispose of the 2 on their sides and keep the rest in the bucket
3. Split them into a larger aerated isolation container with fresh same-temperature water, and protect the wounds with a 0.5% salt bath
4. Torn fins do not heal, so return them all to the river

AnswerC. Split them into a larger aerated isolation container with fresh same-temperature water, and protect the wounds with a 0.5% salt bath

ExplanationKeeping 15 fish overnight in a small bucket combined oxygen depletion, poor water and collisions, injuring every fish. Fish on their sides can recover if the gills are moving. First split them, at lower density, into a larger isolation container with aeration and fresh same-temperature water, protect the wounds with a 0.5% salt bath, and keep it dark and quiet. Going straight into the main tank introduces pathogens to the residents, and the injured fish get chased and worsen. Disposing of the fish on their sides is premature, and continuing in the bucket invites another oxygen crash. Releasing collected fish is strictly forbidden because of genetic contamination and pathogen introduction. Torn fins regrow in clean water, but if cloudiness or cotton appears, switch to columnaris or water-mold treatment.

IssuesToo many fish were collected, without preparing an isolation container to hold them. The owner did not realize that leaving fish in a bucket overnight causes oxygen depletion and poor water, and took more than could be kept.

ImprovementLimit collection to what can be kept (5–8 fish in a 45 cm tank) and prepare an aerated isolation container in advance so fish can be moved as soon as you get home. Plan the collection day as one job that ends with temperature-matching in the isolation container.

An August night at 8 p.m. Arriving home, the 60 cm tank's water level has dropped by half and the floor is flooded. A hose on the external filter had come off and drained the tank. The remaining water is 32°C; 8 pale chub are gasping at the surface and one is on its side. The filter has run dry and stopped.

What is the most appropriate first action?

1. Unplug the filter to prevent electric shock, add aeration, and slowly top up with dechlorinated water close to (slightly below) the tank temperature
2. Run tap water straight from the faucet at full force to restore the level
3. Move all the fish to a bucket and completely wash out the tank
4. Clean the floor first and deal with the fish afterward

AnswerA. Unplug the filter to prevent electric shock, add aeration, and slowly top up with dechlorinated water close to (slightly below) the tank temperature

ExplanationA flooded floor combined with powered equipment risks electric shock and an electrical fire, so first unplug the filter with dry hands. Next, add aeration to secure oxygen, then slowly restore the level with dechlorinated water that is slightly cooler (within 2–3°C) than the 32°C tank water, diluting it. This improves oxygen, temperature and water quality all at once. Chlorinated tap water added directly kills both fish and filter bacteria. Moving everyone to a bucket worsens oxygen depletion through crowding, and washing out the tank destroys the bacteria. Floor cleaning can wait until the fish are dealt with. If the fish on its side still has moving gills after the water change, keep it dark and quiet to recover; if it does not move for hours, consult a vet who treats fish. The filter may be damaged from running dry, so check it before restarting.

IssuesThe owner did not notice the loose hose connection on the external filter and did not understand that it would keep draining once it came off. There was no preparation for the combined risk of rising temperature and oxygen depletion while away in summer.

ImprovementSecure hose connections with clamps and make monthly inspections a habit. Place the external filter below the tank and leave slack in the hoses. In summer, install a cooling fan and a backup air pump, and protect the floor and power outlets with a leak detector or waterproof mat.

A July night, back from a 2-day trip. The breaker had tripped, and the filter and aeration had been off for more than a full day. All 10 bitterlings and gobies in the 45 cm tank are at the surface working their gills fast; the water is slightly cloudy and smells. The water is 28°C. Resetting the breaker will restart the equipment.

What is the most appropriate first action?

1. Reset the breaker, restart the filter, and let the accumulated waste flush into the tank at once
2. Restart only the aeration first; empty the water from the filter before restarting it, and change 1/3 to half the water with same-temperature water
3. Just restore the equipment; they will recover in a few hours, so do nothing else
4. Return all the fish to the river and start the tank over from scratch

AnswerB. Restart only the aeration first; empty the water from the filter before restarting it, and change 1/3 to half the water with same-temperature water

ExplanationInside a filter that has been stopped for a long time, oxygen runs out, the media rot, and toxic water builds up. Restarting it as is flushes that foul water into the tank at once and can poison the fish to death. First restart only the aeration to relieve the oxygen shortage; empty the filter and rinse the media in tank water before restarting it. At the same time, change 1/3 to half the water with same-temperature water to reduce the odor and cloudiness. Simply restoring the equipment does not fix the water quality. Releasing collected fish is strictly forbidden because of genetic contamination and pathogen introduction. If gasping continues after the water change or any fish lies on its side, consult a vet who treats fish.

IssuesThere was no safeguard against a power outage during the trip, and the risk of oxygen depletion when aeration stops was not considered. Knowledge that restarting a long-stopped filter as is can be dangerous was also lacking.

ImprovementBefore a trip, put the equipment on a separate circuit and install a battery-powered air pump that starts automatically on power loss. Check the breaker capacity, and in summer cut back on heating and feeding. Arrange for someone to check in while away, and write down the filter restart procedure.

A February afternoon. The tank water was dirty, so half of it was changed with 8°C tap water; dechlorinator was added but the temperature was not matched. The water, previously 18°C, plunged to 12°C, and right afterward 3 dark chub went onto their sides and the pale chub are swimming unsteadily. The gills are moving.

What is the most appropriate first action?

1. Use a heater to bring it straight back to 18°C
2. Mix in lukewarm water little by little, returning to 18°C over about an hour, and aerate in the dark and quiet
3. Shake the fish on their sides by hand to make them swim
4. Leave them as they are until they adjust to the cold water

AnswerB. Mix in lukewarm water little by little, returning to 18°C over about an hour, and aerate in the dark and quiet

ExplanationA sudden 6°C drop causes temperature shock even in Japanese freshwater fish, leading to fish on their sides and unsteady swimming. With the gills moving, recovery is possible, but a sudden change on the way back up is just as harmful: mix in lukewarm water little by little, returning to the original 18°C over about an hour, and keep it dark and quiet with aeration for oxygen. Heating it straight back delivers a second shock. Shaking by hand damages the skin and adds stress. Left as is, some fish will die of cold shock. After recovery, watch for white spot or water mold for several days. Any fish still on its side after several hours should be seen by a vet who treats fish.

IssuesTemperature matching was skipped at the water change, without checking how cold winter tap water is. The basic rule of no more than 2°C change per day was not ingrained, nor was the habit of checking with a thermometer.

ImprovementAlways measure water for changes with a thermometer and match it to within 1–2°C of the tank before adding. In winter, set the water out the day before or adjust with lukewarm water. Limit changes to 1/3 and avoid changing half or more.

A July evening. A 5-year-old, thinking the fish "looked hungry," dumped an entire bag of pellets into the 45 cm tank of bitterlings and crucian carp. The surface is covered with food and a large amount has sunk to the bottom. The fish keep eating, and the water is 27°C. It was discovered 30 minutes after the food went in.

What is the most appropriate first action?

1. Wait for the fish to eat it all, and skip tomorrow's feeding if any is left
2. Dump all the tank water and replace it with fresh tap water
3. Move the fish to a separate bucket and wash out the main tank
4. Remove all the floating and sunken food with a net and pipette, change 1/3 of the water with same-temperature water, and skip feeding the next day

AnswerD. Remove all the floating and sunken food with a net and pipette, change 1/3 of the water with same-temperature water, and skip feeding the next day

ExplanationA large amount of food rots within hours and produces ammonia; in warm summer water it causes poisoning or oxygen depletion overnight. The fish also overeat and get indigestion. First remove the food thoroughly, floating food with a net and sunken food with a pipette or hose, then change 1/3 of the water with same-temperature dechlorinated water to reduce dissolved organic matter. Skip feeding the next day and test ammonia and nitrite for several days. Letting them eat it all causes both indigestion and water deterioration. A full change without dechlorinator kills the fish through chlorine and loss of bacteria. Moving them to a bucket and washing out the tank also destroys the bacteria and recreates new-tank poisoning. If there is gasping or any fish on its side the next day, change the water again and consult a vet who treats fish.

IssuesFood was kept within a child's reach, and the rule of feeding only small amounts had not been shared with the family. The family did not realize that overfeeding is the main cause of disease and poor water in Japanese freshwater fish.

ImprovementStore food out of children's reach, and have an adult feed 1–2 times a day, only what is eaten in 2 minutes. When letting a child feed, measure the amount into a small dish first. Keep a pipette and test strips on hand to check the water after an accident.

An August afternoon, collecting bitterlings in a channel connected to a pond. Mixed in with the catch is a 4 cm fish with a dark blue-black spot behind the gill cover. A field guide confirms it is a juvenile bluegill. The child says, "It's cute, I want to take it home and keep it."

What is the most appropriate action?

1. Do not take it home alive; deal with it on the spot according to the local government's eradication rules, and do not put it in the bucket
2. It is fine while small, so keep it at home with the bitterlings
3. Put it in the bucket, take it home, and release it in another pond nearby
4. Since it cannot be kept, take it home and then release it in a nearby river

AnswerA. Do not take it home alive; deal with it on the spot according to the local government's eradication rules, and do not put it in the bucket

ExplanationThe bluegill is a designated Invasive Alien Species under the Invasive Alien Species Act; transporting, keeping or releasing it alive is prohibited as a rule, and violations carry prison terms or fines. Merely putting it in the bucket to take home counts as illegal transport, so take it out of the water on the spot and deal with it according to the local government's eradication rules. Keeping it is prohibited even as a juvenile, and once grown it preys on bitterlings. Releasing it in another pond or river spreads an invasive species, causes serious ecosystem damage, and is the most serious violation. "Releasing it because we can't keep it" is wrong even for native fish, since it causes genetic contamination and introduces pathogens; never release collected fish of any species. Before collecting, find out whether designated invasive species occur in that water.

IssuesThe possibility of invasive species at the collection site was not considered, and there was no habit of checking the species of every fish in the net. The owner did not know the Invasive Alien Species Act's rules and nearly gave in to the child's wishes.

ImprovementBefore collecting, check the local government website for invasive species in the target water and learn to identify bluegill, largemouth bass and mosquitofish. Check every fish in the net one by one and never put a designated invasive species in the bucket. Explain to children beforehand that some fish cannot be taken home.

A September night. Rushing in at a noise, the family cat has pushed the light lid aside and has a front paw in the tank, and a pale chub has jumped out onto the floor. It has a shallow scratch from the cat's claws on its flank and several scales are missing. The fish is flapping and its gills are moving.

What is the most appropriate first action?

1. Scold the cat, then grab the fish with dry hands and put it back in the tank
2. Wash the wound with antiseptic, then return it to the main tank
3. Pick it up with wet hands, move it to an isolation container of same-temperature tank water, protect the wound with a 0.5% salt bath, and replace the lid with a heavy one
4. Decide the wound will not heal and dispose of the fish

AnswerC. Pick it up with wet hands, move it to an isolation container of same-temperature tank water, protect the wound with a 0.5% salt bath, and replace the lid with a heavy one

ExplanationA fish that is flapping with moving gills has a good chance of recovery. Pick it up gently with wet hands, move it to an isolation container of same-temperature tank water, protect the wound with a 0.5% salt bath, and add a slime-coat protectant at the specified dose. Cat-claw wounds are prone to bacterial infection, so watch for red swelling or cotton-like growth for several days. Dry hands strip the slime coat, and human antiseptic is toxic to fish. Returned directly to the main tank, it would be pecked by other fish and the wound would spread. A shallow wound heals, so disposal is inappropriate. To prevent recurrence, switch to a heavy glass lid or a fixed lid and keep objects off the top of the tank so the cat cannot climb on it. If the wound becomes red and gouged, the fish stops eating, or cotton appears, consider disease treatment and a vet visit.

IssuesThe owner did not anticipate the cat's interest in the tank and used a lid that was merely set on top. Safety measures for keeping a cat and fish in the same room were inadequate.

ImprovementUse a heavy glass lid or a lid that can be fixed in place, and close all gaps. Arrange things so the cat cannot get on top of the tank, and separate rooms if possible. Keep a slime-coat protectant and isolation container on hand, and share the rescue procedure for a fish on the floor with the family.

A March midday, while moving furniture, a 45 cm tank sitting on an unstable stand tipped over; the glass broke, the water poured out, and 8 dark chub and bitterlings are scattered across the floor. Broken glass is everywhere. Some fish are flapping, others are not moving. A bucket and a store of dechlorinated water are at hand.

What is the most appropriate first action?

1. Clear all the glass from the floor before gathering the fish
2. Decide the motionless fish are dead and put only the moving ones in the bucket
3. Gather all the fish in a bucket filled with tap water
4. Watching for glass, gather all the fish with wet hands into a bucket of the stored dechlorinated water, add an air pump, and keep it dark and quiet

AnswerD. Watching for glass, gather all the fish with wet hands into a bucket of the stored dechlorinated water, add an air pump, and keep it dark and quiet

ExplanationFish survive only a few minutes to perhaps ten-odd minutes out of water, so the top priority is getting them back into water. Avoiding the glass in thick-soled shoes or slippers, gather every fish, including the motionless ones, with wet hands into a bucket of the stored dechlorinated water; if the gills are moving there is a chance of recovery. Add an air pump for oxygen and keep it dark and quiet to calm them. Clearing the glass first lets the fish dry out and die. Abandoning the motionless fish throws away the chance to save them. Chlorinated tap water damages the gills and finishes off weakened fish. Once the fish have settled, check for skin injuries and protect them with a salt bath. Set up the new tank with matched temperature and water quality. Any fish still motionless after several hours should be seen by a vet who treats fish.

IssuesThe tank was on an unstable stand, and the fact that it weighs over 40 kg when full was taken lightly. Safety around the tank was not checked while moving furniture, and no thought was given to a container and water for emergencies.

ImprovementPlace the tank on a sturdy, purpose-made stand and secure it with an anti-seismic mat and tip-over protection. Keep stored dechlorinated water, a spare bucket and a battery-powered air pump on hand. Before moving furniture or doing construction, move the fish temporarily to a safe container.

An April morning. The thermostat on a heater in use since fall has failed, and the thermometer in the 60 cm tank of bitterlings and gobies reads 35°C. The fish are gaping near the surface and the gobies are limp on top of the stones. The heater is hot to the touch and still heating.

What is the most appropriate first action?

1. Unplug the heater, increase aeration, and lower the temperature over about an hour with small changes of dechlorinated water 3–4°C cooler than the tank
2. Unplug the heater and add a large amount of ice to drop it to 25°C at once
3. Leave the heater plugged in and just run a cooling fan
4. Move all the fish to a bucket of cold tap water to cool them fast

AnswerA. Unplug the heater, increase aeration, and lower the temperature over about an hour with small changes of dechlorinated water 3–4°C cooler than the tank

Explanation35°C is close to lethal for Japanese freshwater fish, and dissolved oxygen is extremely low. First unplug the heater, the source of heat, and increase aeration to secure oxygen. Lower the temperature over about an hour with small changes of dechlorinated water 3–4°C cooler. Rapid cooling is a second shock, so a large amount of ice or a bucket of cold tap water is dangerous, and the chlorine makes it fatal. Running a cooling fan with the heater still plugged in is pointless as it keeps heating. Do not take the unplugged heater out of the water (it can crack in air); remove it once it has cooled. If any fish is still on its side after the water change, keep it dark and quiet to recover; if it does not move for hours, consult a vet who treats fish. Discard the failed heater and replace it with an integrated-thermostat model with dry-run protection.

IssuesThe heater's service life was not tracked and there was no safety device against overheating. The water temperature was not checked every morning even in spring, so nothing was noticed until it reached 35°C.

ImprovementReplace heaters every 1–2 years and use models with overheat protection. Check the thermometer every morning and install a digital thermometer with a temperature alarm. In spring, remove the heater once the water warms and switch to unheated keeping.

Late July, midday. A 45 cm tank by the window has reached 31°C. 6 pale chub and dark chub are gaping near the surface with rapid gill movement. The room has no air conditioning and no cooling fan is fitted. Direct sunlight hits the tank and the glass is warm to the touch.

What is the most appropriate first action?

1. Put ice directly in the tank to cool it fast
2. Add cold tap water to half the tank to drop the temperature at once
3. Block the direct sunlight, increase aeration, and float frozen plastic bottles to lower the temperature gradually
4. Fish tolerate heat, so wait until evening

AnswerC. Block the direct sunlight, increase aeration, and float frozen plastic bottles to lower the temperature gradually

ExplanationPale chub and dark chub are stream fish with high oxygen needs, and at 31°C they suffocate from low dissolved oxygen. The safe approach is to first block the direct sunlight to cut off the heat source, increase aeration to secure oxygen, and float frozen plastic bottles to lower the temperature at a rate of 1–2°C per hour. Ice added directly chills locally and causes shock. Half a tank of cold tap water kills the fish through the temperature gap and chlorine. Waiting until evening means some fish die of oxygen depletion. After first aid, fit a cooling fan and move the tank away from the window. If changing water, do it 1/3 at a time with dechlorinated water 2–3°C cooler than the tank. If gasping continues or any fish lies on its side, consult a vet who treats fish.

IssuesThe tank was placed by a sunny window, and the summer risk of rising temperature and oxygen depletion was taken lightly. There was no cooling fan or extra aeration, and no plan for scorching days.

ImprovementPlace the tank out of direct sunlight and fit a cooling fan before summer. Keep frozen plastic bottles ready and cool the tank urgently when the water reaches 30°C. Manage room temperature with air conditioning if possible, and check the thermometer daily.

A September night. A typhoon has cut the power, stopping the filter and aeration on a 60 cm tank. There is no estimate for restoration, and the water is 27°C. The tank holds 12 fish in all, pale chub, bitterlings and gobies, and after 2 hours the fish are starting to gather near the surface. There is no battery-powered air pump at hand, only plastic bottles and a cup.

What is the most appropriate first action?

1. Feed the fish to keep their strength up
2. Do nothing until the power is restored and wait for the filter to run
3. Move all the fish to a small bucket and put it in a cool place in the house
4. Scoop water with a cup and pour it from a height to mix in air, open the lid, and stop feeding

AnswerD. Scoop water with a cup and pour it from a height to mix in air, open the lid, and stop feeding

ExplanationWhen a power outage stops the aeration, a 27°C tank with 12 fish runs out of oxygen within hours. Without a battery pump, the most effective first aid is to scoop water with a cup or plastic bottle and pour it back from a height, agitating the surface to mix in air; repeat every 30 minutes to an hour. Open the lid to increase contact with the air, and stop feeding because it consumes oxygen and fouls the water. Merely waiting for restoration means the fish die. A small bucket has less water and becomes oxygen-depleted even faster. After power returns, empty the foul water from the long-stopped filter before restarting it. If any fish progresses from gasping to lying on its side, support it near the surface with its mouth facing the current and consult a vet who treats fish.

IssuesDespite the typhoon forecast, no battery-powered air pump was prepared for a power outage. The owner did not recognize that 12 fish made the tank prone to oxygen depletion during an outage.

ImprovementAlways keep a battery-powered air pump and spare batteries, and set it up in advance when a typhoon or earthquake warning is issued. Keep fish numbers conservative for the tank size and share the manual oxygenation method with the family. Also write down the procedure for restarting the filter after an outage.

A large earthquake struck late at night. A third of the water spilled from the 45 cm tank, the fish are unharmed, but the power and water supply are out. The local government has issued an evacuation order. At hand are 10 L of stored dechlorinated water and a battery-powered air pump. Pets cannot be taken to the shelter.

What is the most appropriate action?

1. Put the battery-powered air pump in the tank, top up the spilled water with the stored water, put the lid on, and evacuate, putting your own safety first
2. Put all the fish in a bucket and take them to the shelter
3. Release the fish into the river before evacuating
4. Do not evacuate; stay with the fish and keep managing temperature and oxygen

AnswerA. Put the battery-powered air pump in the tank, top up the spilled water with the stored water, put the lid on, and evacuate, putting your own safety first

ExplanationWhen an evacuation order has been issued, human life comes first. With oxygen from the battery-powered air pump, the spilled water topped up from storage and the lid on, the fish can last several days without feeding. Do not feed them; check the water and equipment when you can return. Carrying them to the shelter in a bucket is unrealistic because of oxygen depletion and collisions from jolting in transit and the difficulty of securing water at the shelter, and they would not be admitted anyway. Releasing fish, even collected ones, is strictly forbidden because of genetic contamination and pathogen introduction. Staying behind risks losing your own life in a secondary disaster. After the disaster, check the number of fish, their skin and the water temperature, and empty the water from the long-stopped filter before restarting it.

IssuesHow to handle the fish in a disaster had not been thought through in advance, causing hesitation over evacuating. Stored water and a battery-powered air pump were prepared, but knowledge of how long fish can last without feeding was lacking.

ImprovementDecide in advance that human life comes first, and share with the family a four-point policy for leaving the fish: air pump, top up, lid, no feeding. Keep 10 L or more of stored water, a battery-powered air pump and spare batteries, secure the tank with an anti-seismic mat, and keep the water level slightly lowered.

A June weekend. Bitterlings collected last year from a river in another prefecture have bred using freshwater mussels, and the 45 cm tank now holds over 30. It is overcrowded and the water quality is deteriorating. A family member says, "If we release them in the local river, which is similar to where they came from, they can go back to nature and be happy," and is about to move them to a bucket to release them.

What is the most appropriate action?

1. Not the local river; drive them back to a river in the original prefecture and release them there
2. Release them in the local river. They are the same bitterling, so there is no problem
3. Release the surplus in a nearby pond and keep the rest
4. Do not release them; keep them all by adding tanks or giving them to acquaintances or a specialist shop. If no one wants them, ask the shop about options other than release

AnswerD. Do not release them; keep them all by adding tanks or giving them to acquaintances or a specialist shop. If no one wants them, ask the shop about options other than release

ExplanationBitterlings have genetically distinct local populations in each river, and releasing them in a different river causes genetic contamination through hybridization. Even in their original river, fish bred in a tank or housed with other fish risk introducing pathogens and parasites to wild populations. Releasing into a local river or pond harms the ecosystem even with native species, and may be prohibited by local ordinance. The correct response is to keep them all: add tanks and ask acquaintances or a specialist shop dealing in Japanese freshwater fish about rehoming. If you do not want continued breeding, remove the mussels. The idea that they will be "happy back in nature" is mistaken; captive-raised fish often cannot survive in the wild, and the release itself endangers other fish.

IssuesBreeding was not managed, and the result was to be solved by release. The family did not know the concepts of local populations and genetic contamination and held the misconception that the same species can be returned.

ImprovementMake keeping every fish for life a family rule at collection time, and if breeding, decide how many can be kept and manage when mussels are added. Line up a specialist shop or hobbyist group in advance to take surplus fish. Explain to children why release is prohibited.

A May afternoon, in a narrow spring-fed stream in the Kanto region, a bitterling about 4 cm long with red edging on its dorsal and anal fins and a blue-green sheen is spotted. A local sign reads "Tokyo bitterling habitat – collection prohibited." The child has caught it in a net, put it in the bucket, and says, "I want to keep it."

What is the most appropriate action?

1. Just one is no problem, so take it home and keep it
2. Take a photo, then release it somewhere else
3. Immediately return it to the exact spot on the site, do not collect it, and report any records to the local government or the Ministry of the Environment
4. It is a rare species, so take it home for protection and breed it at home

AnswerC. Immediately return it to the exact spot on the site, do not collect it, and report any records to the local government or the Ministry of the Environment

ExplanationThe Tokyo bitterling is a national Natural Monument (Cultural Properties Protection Act) and is designated a National Endangered Species under the Act on Conservation of Endangered Species. Collecting, keeping or transferring it without a permit is illegal and punishable by prison or a fine. There is no exception even for a single fish. If one ends up in the net, gently return it right away to the exact spot it came from; releasing it elsewhere also disturbs the habitat and must be avoided. Well-meaning "keeping for protection" is also illegal, and amateur keeping does nothing to conserve the population. If you have a record of where and how it was caught, reporting it to the local government or the regional office of the Ministry of the Environment helps conservation. Likewise the deepbody bitterling (Itasenpara) and Suigen bitterling are protected under the Act on Conservation of Endangered Species. Make it a habit to check signs and the local government website for protected species before collecting.

IssuesDespite the no-collection sign, it was not checked before collecting began. The owner did not know that fish are designated under the Act on Conservation of Endangered Species or as Natural Monuments, and could not stop the child from collecting.

ImprovementBefore collecting, check on-site signs and the local government website for protected species and no-collection zones. Many bitterlings are rare, so learn to identify them from a field guide and do not take any you cannot identify. Teach children beforehand that there are fish that must not be caught.

An August night. A 12 cm crucian carp foraging by poking at the substrate has taken a small piece of gravel into its mouth and cannot spit it out; its mouth is stuck open and it is working its gills in distress. It is floating at the surface, rocking its body. The gravel is slightly smaller than the fish's mouth.

What is the most appropriate first action?

1. Push tweezers deep into the mouth and pull it out by force
2. It will swallow it eventually, so leave it and watch
3. Take the fish out and press its mouth with your fingers to push the gravel out
4. Observe for a few minutes; if it cannot free it, support the fish with wet hands and, if the mouth is open, gently remove the visible gravel with blunt-tipped tweezers

AnswerD. Observe for a few minutes; if it cannot free it, support the fish with wet hands and, if the mouth is open, gently remove the visible gravel with blunt-tipped tweezers

ExplanationCrucian carp forage by poking at the substrate and sometimes get gravel close to their mouth size stuck. Most spit it out within minutes on their own, so observe briefly first. If it cannot, support the fish in the water with wet hands and, if the mouth is open and the gravel visible, gently remove it with blunt-tipped tweezers. Pushing deep or pulling by force injures the mouth and pharynx, leading to infection and inability to eat. Left too long, it suffocates or weakens. Pressing the mouth with fingers damages bone and scales. If the gravel is lodged out of sight, the mouth will not close after removal, or the fish cannot eat, consult a vet who treats fish the same day. Afterward add a slime-coat protectant and watch appetite and mouth movement for several days.

IssuesSmall gravel close to the carp's mouth size was used as substrate, with no consideration for its habit of foraging in the substrate. The possibility of ingestion was not anticipated and mouth size was not compared with the gravel.

ImprovementFor crucian carp and bitterlings, switch to gravel too large to fit in the mouth or to fine sand that can be spat out if taken in. Choose sinking food of a size that suits the mouth and keep blunt-tipped tweezers on hand. Review the substrate as the fish grow.

A July night. Aerosol insecticide was sprayed in the room with the tank to kill a mosquito. The lid was half open. A few minutes later the pale chub and bitterlings are swimming around frantically, writhing as if in convulsions, and one is on its side. An oily film is floating on the surface.

What is the most appropriate first action?

1. Open the windows to ventilate, lift the film off the surface with paper towels, change half the water with same-temperature dechlorinated water, and increase aeration
2. Do nothing; keep it dark and wait for the fish to settle
3. Pour large amounts of tap water straight from the faucet to dilute the insecticide
4. Just add activated carbon to the filter without changing the water

AnswerA. Open the windows to ventilate, lift the film off the surface with paper towels, change half the water with same-temperature dechlorinated water, and increase aeration

ExplanationMost insecticides are extremely toxic to fish, and the chemicals that land on the surface dissolve in and cause neurological symptoms. First ventilate to reduce the chemicals in the air, lift the oily film off with paper towels to stop further dissolving, change half the water with same-temperature dechlorinated water to lower the concentration, and increase aeration to ease the strain on the gills. Activated carbon is useful after the water change, but is not enough on its own. Waiting lets the neurological symptoms progress to death. Chlorinated tap water added directly only adds more poison. Isolate any fish still convulsing after the water change, and if symptoms persist the next day consult a vet who treats fish. From now on, never use insecticides or foggers in the room with the tank.

IssuesThe owner did not know that using insecticide in the room with the tank is deadly to fish, and sprayed with the lid half open. There was no awareness that mosquito coils and foggers are just as dangerous.

ImprovementMake it a family rule never to use insecticides, foggers or mosquito products in the room with the tank. If use is unavoidable, seal the tank and move it to another room, or ventilate thoroughly after spraying before bringing it back. Keep activated carbon and spare stored water on hand for emergency water changes.

An April midday, as a "big clean," nearly all the water in a 45 cm tank was dumped, tap water poured straight in, and the 7 dark chub and gobies returned. 30 minutes later the owner realized the dechlorinator had been forgotten. The fish are working their gills hard, their slime coat is peeling off in cloudy white patches, and they are wobbling near the surface.

What is the most appropriate first action?

1. Do nothing and wait for the fish to adjust
2. Dump all the water now and replace it with fresh tap water again
3. Immediately add dechlorinator at the specified dose and mix it through, increase aeration, and add a slime-coat protectant at the specified dose
4. Take all the fish out and wipe the white cloudiness off their bodies with a dry towel

AnswerC. Immediately add dechlorinator at the specified dose and mix it through, increase aeration, and add a slime-coat protectant at the specified dose

ExplanationChlorine in tap water directly damages the gills and the slime coat, causing it to peel and impairing gill function. Chlorine is neutralized instantly by a dechlorinator, so the best response is to add it immediately at the specified dose and mix it through, increase aeration to ease the strain on the gills, and protect the skin with a slime-coat protectant. Waiting lets the chlorine keep acting. Replacing with tap water again adds more chlorine and destroys the filter bacteria. Wiping with a towel strips what slime coat remains. Since the full change has wiped out the bacteria, test ammonia and nitrite for several days and change water as needed. If the gills stay white or fish remain on their sides, consult a vet who treats fish.

IssuesA full water change was done as a "big clean," without understanding the importance of filter bacteria or the toxicity of chlorine. Adding dechlorinator was not built into the water-change routine.

ImprovementLimit water changes to 1/3 and never do a full change. Fix the procedure so dechlorinator goes into the water-change bucket first, and keep it next to the tank. Only rinse filter media in tank water, to protect the bacteria.

A May afternoon. Bothered by algae, the owner moved the fish to a bucket, washed the tank and filter media with dish detergent, rinsed, refilled, and returned the fish. An hour later fine foam is forming on the surface, 8 bitterlings are gasping at the surface, and 2 are rigid and on their sides.

What is the most appropriate first action?

1. The foam will disappear in time, so watch and wait
2. Remove only the surface foam and keep the fish in the tank
3. Move the fish back to the bucket and wash the tank thoroughly with detergent again
4. Move the fish to a separate container of same-temperature dechlorinated water with aeration, and rinse the tank and media repeatedly with large amounts of water, no detergent

AnswerD. Move the fish to a separate container of same-temperature dechlorinated water with aeration, and rinse the tank and media repeatedly with large amounts of water, no detergent

ExplanationThe surfactants in dish detergent destroy gill function, and even a trace residue kills fish. The surface foam is proof of residue: immediately move the fish to a separate container of same-temperature dechlorinated water with aeration, away from the poison. Rinse the tank and media repeatedly with large amounts of water, no detergent, until no foam appears. The detergent has killed the bacteria in the media, so replace it or re-cycle it with tank water. Watching lets the poisoning progress. Removing the foam leaves the dissolved detergent. Washing with detergent again increases the residue. Even the rigid fish can recover if the gills are moving, so keep them dark and quiet in the separate container. Any fish that does not recover should be seen by a vet who treats fish. Test the water for several days after restarting.

IssuesThe owner did not know the basic rule that detergent must never be used on a tank or filter media. Algae removal was done for looks, without any thought for protecting the filter bacteria.

ImprovementClean the tank without detergent, removing algae physically with a sponge and scraper, and only rinse filter media in tank water. Keep aquarium tools separate from kitchen items and make sure the family knows. Prevent algae by controlling lighting hours and feeding.

Mid-June, evening. Believing "natural water is better for fish," the owner drew water from an irrigation channel beside a freshly planted rice paddy and added it as 1/3 of the tank. The next morning, 3 of 9 pale chub and bitterlings are on their sides convulsing, and the rest are gaping at the surface. Pesticide was recently sprayed around the channel.

What is the most appropriate first action?

1. Change half the water with same-temperature dechlorinated water, increase aeration, and add activated carbon to the filter
2. Add more water from the same channel to dilute it
3. It is natural water, so there is no problem; decide they will adjust in a few days and watch
4. Return the fish to the channel to recover in a natural environment

AnswerA. Change half the water with same-temperature dechlorinated water, increase aeration, and add activated carbon to the filter

ExplanationRight after rice planting, irrigation channels can carry pesticides and herbicides that are neurotoxic to fish. Convulsions and fish on their sides are signs of poisoning: change half the water with same-temperature dechlorinated water to lower the concentration, use aeration to ease the strain on the gills, and add activated carbon to the filter to adsorb the dissolved chemicals. More water from the same channel adds more poison. Waiting a few days means more fish die. Returning collected fish to the channel is a release and is strictly forbidden because of genetic contamination and pathogen introduction. Besides pesticides, channel water brings in parasites, pathogens and ammonia, so never use it as tank water. Isolate any fish still convulsing after the water change, and if there is no improvement the next day consult a vet who treats fish.

IssuesThe belief that natural water is good for fish left no thought for pesticides, parasites and pathogens. The owner did not know that pesticides are sprayed at planting time and drew the water without checking the surroundings.

ImprovementAlways use dechlorinated tap water for the tank and never add river or channel water. Do not mix collection water from the bucket into the main tank either. Keep activated carbon and stored water on hand so water can be changed immediately in a poisoning.

An October morning. Two weeks ago, the owner did a layout change and water change bare-handed with a hangnail on one finger. That finger has become red and swollen with a lump that hurts when pressed. It has not healed, and the swelling is gradually spreading toward the wrist. There is no fever, but the lumps are multiplying.

What is the most appropriate action?

1. Put on an adhesive bandage and wait for it to heal on its own
2. Pop the lump with a needle to drain the pus
3. See a dermatologist and be sure to tell the doctor about contact with aquarium water
4. The fish in the tank are the cause, so dispose of all of them

AnswerC. See a dermatologist and be sure to tell the doctor about contact with aquarium water

ExplanationNontuberculous mycobacterial infection (fish tank granuloma), contracted through a wound exposed to tank water or fish, characteristically produces red lumps that spread in a line from the wound up the arm; it rarely heals on its own and ordinary antibiotics work poorly. See a dermatologist and say that you touched aquarium water and keep fish, which speeds the diagnosis. Left under a bandage it worsens and requires long treatment. Popping the lump spreads the infection. Disposing of the fish does not cure your infection; it is a zoonosis that proper hand washing and gloves prevent. From now on, wear rubber gloves when you have any cut and wash with soap after work. People with weakened immunity and children should be especially careful.

IssuesTank work was done bare-handed with a hangnail, without knowing that tank water harbors infectious organisms. When symptoms appeared they were not linked to a fish-related infection, delaying the doctor's visit.

ImprovementMake rubber gloves standard for tank work, with or without cuts, and wash with soap afterward. When you have a cut on your hand, ask a family member to do the work. Agree as a family that if lumps or swelling appear, you see a dermatologist and mention keeping fish.

An August evening, with pale chub and loaches collected from the river in front of them, the family suggests, "They're fresh, let's eat them as sashimi or live." They insist the fish are safe because they came from a clean river. There is a freezer, but there are doubts about eating them raw. The child also wants to try.

What is the most appropriate action?

1. They are from a clean river, so eat them raw while fresh
2. Marinating in vinegar kills parasites, so eat them pickled
3. A light rinse with salt makes them safe, so make sashimi
4. Do not eat them raw; if eating, cook thoroughly to the center. If keeping them, do not eat them

AnswerD. Do not eat them raw; if eating, cook thoroughly to the center. If keeping them, do not eat them

ExplanationJapanese freshwater fish can carry the larvae of Metagonimus flukes (common in pale chub, Japanese dace and others) and Gnathostoma (loaches, snakeheads and others), which infect people who eat them raw. Gnathostoma migrates under the skin causing swellings and can enter the eye or brain. Metagonimus causes abdominal pain and diarrhea. Parasites are present even in clean rivers and cannot be seen. Vinegar and salt do not kill them; cooking thoroughly to the center is the only reliable method. Home freezers often do not reach the required temperature and time. Fish collected for keeping should not be eaten, because of medications and bacteria in the tank water. If migrating swellings under the skin or abdominal pain follow raw consumption, tell the doctor about eating raw freshwater fish.

IssuesThe family held the misconception that fresh and clean-river means safe, with no knowledge of the parasites specific to freshwater fish. They believed the folk notion that vinegar or salt kills parasites.

ImprovementAgree as a family that freshwater fish are always cooked, with no live eating or sashimi. Keep fish collected for the aquarium separate from food. Learn the symptoms of parasitic infection from raw freshwater fish and, if it happens, tell the doctor about the raw consumption.

A September evening. 4 dark chub collected from the river have been brought home. The main tank holds pale chub and bitterlings kept for a year. The child says, "I want to see them swim together soon," and is about to pour the collected fish into the main tank together with the bucket water. No abnormalities are visible on the new fish's skin so far.

What is the most appropriate action?

1. Quarantine them in a separate container for 1–2 weeks, inspecting skin, fins and gills daily, before adding them. Do not put the bucket water in the main tank
2. There is nothing wrong with their skin, so add them to the main tank, bucket water and all
3. Net just the fish into the main tank and discard the bucket water
4. Add fish medication to the main tank preventively, then add them

AnswerA. Quarantine them in a separate container for 1–2 weeks, inspecting skin, fins and gills daily, before adding them. Do not put the bucket water in the main tank

ExplanationWild-caught fish often carry Ich, anchor worms, fish lice and bacteria even when they look normal. The basic rule is to quarantine them in a separate container for 1–2 weeks, inspecting skin, fins and gills daily, and add them only if no problems appear. The collection bucket water contains river parasites and pathogens, so not a drop goes into the main tank. Moving only the fish still brings in pathogens that are in their incubation period. Preventive fish medication stresses the main tank's filter bacteria and resident fish and is of uncertain effect. If problems appear during quarantine, treat in the quarantine container and protect the residents. Match the quarantine container's temperature and add aeration and a hideout to reduce stress.

IssuesThere was no awareness of pathogens carried by collected fish, and the judgment "if they look healthy they're safe" was nearly made. No quarantine container was prepared, and things were heading toward giving the child what they wanted.

ImprovementBefore a collecting trip, prepare a quarantine container, air pump and hideout, and make "2 weeks of quarantine before adding" a family rule. Keep observation records during quarantine, and if anything is abnormal, keep the main tank and its equipment separate.

A November morning. One bitterling in the 60 cm tank is dead, with white cotton-like growth on its skin. It was swimming normally until yesterday. The remaining 10 bitterlings and crucian carp look fine for now. The net is shared with the quarantine tank next door. The water is 14°C and was changed a week ago.

What is the most appropriate first action?

1. Leave the dead fish for the others to eat
2. Throw the dead fish into the garden pond to return it to nature
3. Just remove the dead fish and keep using the net and tools as they are
4. Remove the dead fish at once and dispose of it with the household waste, keep nets and tools separate per tank or disinfect them in boiling water, and observe the remaining fish for several days

AnswerD. Remove the dead fish at once and dispose of it with the household waste, keep nets and tools separate per tank or disinfect them in boiling water, and observe the remaining fish for several days

ExplanationA dead fish is a breeding ground for bacteria and water mold; left in place, the water deteriorates and pathogens spread through the tank. Remove it at once and dispose of it as household waste according to local rules. Throwing it into a pond, river or channel spreads pathogens and parasites into the wild and is prohibited just like releasing fish. Shared nets and tools carry disease to other tanks, so keep them separate per tank or disinfect them in boiling water. Watch the remaining fish for several days for cotton-like growth or loss of appetite, and change 1/3 of the water with same-temperature water to restore quality. Letting the others eat it means they take in the pathogens. If cotton or white spots spread to several fish, begin isolation and treatment, and if the cause is unclear consult a vet who treats fish.

IssuesThe owner did not know how to dispose of a dead fish or the risk of spreading infection through shared tools. The fish looked normal until the day before, but the early white cotton was not caught in observation.

ImprovementDedicate nets, pipettes and buckets to each tank and disinfect them in boiling water after use. Check the headcount and skin of every fish each morning and isolate immediately on any abnormality. Confirm the local rules for disposing of dead fish and never discard them outdoors.

Late on a January night. An 18 cm crucian carp in an unheated 60 cm tank is found lying on its side on the bottom. The water is 7°C. Its color is slightly pale and it does not appear to move at all. It was not fed today. The other fish are still on the bottom but moving their gills slowly.

What is the most appropriate first action?

1. It is not moving, so decide it is dead, remove it at once, and dispose of it
2. Net it out at once into a separate container and warm it up fast with a heater
3. Shine a light on it, check for gill-cover and eye movement, and bring a finger close to see if it reacts
4. Tap the tank to startle it and see whether it moves

AnswerC. Shine a light on it, check for gill-cover and eye movement, and bring a finger close to see if it reacts

ExplanationIn cold water a crucian carp's metabolism drops drastically and gill movements become slow and small. First shine a light on it and check whether the gill covers move even slightly and whether the eyes move, then bring a finger close gently to see if it reacts. If the gills are moving it is alive, possibly in cold shock or weakened; keep it dark and quiet with gentle aeration and watch it while avoiding sudden temperature changes. Disposing of it just because it is not moving risks throwing away a living fish. Netting it out and warming it fast kills it by temperature shock. Tapping the tank stresses every fish. If the gills do not move at all, the eyes are cloudy and the body is stiff, judge it dead and remove it at once. If it does not recover, consult a vet who treats fish the next morning.

IssuesUnaware of fish physiology in cold water, the owner nearly mistook stillness for death. Not knowing how to check for life by gill-cover and eye movement, there was a risk of rashly warming or disposing of the fish.

ImprovementDecide in advance to check for life by three points: gill-cover movement, eye movement and response to a finger. Know that normal gill movement is slow in winter and judge in light of the water temperature. Around 7°C, withhold food and avoid sudden changes.

An April morning. A dark chub found on the floor was returned to the tank but sinks on its side and cannot swim on its own. The gills are moving slightly. The body had half dried out, but some slime coat remains. The tank is 25°C and the other fish are coming to peck at it. It may have been several hours before it was found.

What is the most appropriate next action?

1. Move it to a separate container of same-temperature tank water, support it near the aeration with its mouth facing the current, keep it dark, and do not touch it until it moves
2. Other fish pecking is common, so leave it in the main tank
3. It is on its side, so decide it is dead and remove it
4. Rock its body back and forth by hand to make it swim and stimulate breathing

AnswerA. Move it to a separate container of same-temperature tank water, support it near the aeration with its mouth facing the current, keep it dark, and do not touch it until it moves

ExplanationAs long as the gills are moving, revival is possible. Chest compressions are impossible for fish, so the only life-saving measure is to pass water over the gills. Move it to a separate container of same-temperature tank water where other fish cannot peck it, support it near the aeration with its mouth facing the current so oxygenated water flows over the gills, and keep it dark and quiet without touching until it begins to swim on its own. Left in the main tank it gets pecked, gains more injuries, and a weakened fish cannot recover. Lying on its side is not death. Rocking it by hand damages the skin and the stress is counterproductive. After recovery, add a slime-coat protectant at the specified dose and watch for white cotton for several days. If the gills begin to stop after several hours, or it is still on its side the next day, consult a vet who treats fish.

IssuesNot knowing the rescue procedure after returning a jumped-out fish to water, the owner left it in the main tank. The habit of other fish pecking a weakened fish was underestimated, and no isolation container was ready.

ImprovementKeep an isolation container and air pump on hand and share a four-point rescue procedure with the family: separate container, current, dark and quiet, no touching. Close lid gaps to prevent jumping in the first place. Keep a slime-coat protectant in stock.

A May night. A pale chub snagged its tail fin on the tank's driftwood; the fin is badly torn and red blood is seeping from the base in a thin stream through the water. The fish can swim but is in a corner with clamped fins. Other fish are approaching and pecking at it. The driftwood still has sharp branches.

What is the most appropriate first action?

1. Take the fish out and press a tissue against the wound to stop the bleeding
2. Apply a human styptic or antiseptic to the wound
3. The bleeding will stop on its own, so watch it in the main tank
4. Move it to an isolation container of same-temperature tank water, protect it with a slime-coat protectant and a 0.5% salt bath, and see a vet if bleeding continues

AnswerD. Move it to an isolation container of same-temperature tank water, protect it with a slime-coat protectant and a 0.5% salt bath, and see a vet if bleeding continues

ExplanationFin bleeding usually stops on its own in clean water, and applying pressure to a fish means taking it into the air, which damages the slime coat and gills and is more dangerous. In a tank where other fish peck at it, the wound spreads and becomes infected, so move it to an isolation container of same-temperature tank water, protect the wound with a slime-coat protectant and a 0.5% salt bath, and keep it dark and quiet. Human styptics and antiseptics are toxic to fish and wash off in water. Watching in the main tank means it gets pecked and worsens. Torn fins regrow in clean water, but if the bleeding does not stop within hours, if there is bleeding from the body, or if cloudiness or cotton appears, consult a vet who treats fish. Remove the sharp edges from the driftwood that caused it, or take it out.

IssuesSharp driftwood was placed in the tank without anticipating that a startled pale chub could snag itself. The owner did not know that fish bleeding cannot be stopped by pressure as in people, and no isolation container was ready.

ImprovementUse only driftwood and stones with the edges removed and keep a layout where fish are not injured even if they collide. Keep an isolation container, slime-coat protectant and salt on hand and learn the procedure of protecting the fish in water and waiting for bleeding to stop. Isolate at once if other fish peck at it.

A November afternoon. A crucian carp with spreading red holes on its flank is to be taken to a veterinary clinic that treats fish, 40 minutes each way. At hand are tank water, plastic bags, a foam box, a food container and tap water. The outside temperature is 12°C and the tank is 20°C. The fish can swim but is sluggish.

What is the most appropriate transport method?

1. Put the fish in the food container with tap water and set it on the car's passenger seat
2. Fill a bucket with tank water and hold it in the car without a lid
3. Seal the fish in a bag of tank water with plenty of air, place it in the foam box to keep the temperature stable, and carry it dark and without shaking
4. Wrap the fish in a wet towel, put it in a bag, and carry it without water

AnswerC. Seal the fish in a bag of tank water with plenty of air, place it in the foam box to keep the temperature stable, and carry it dark and without shaking

ExplanationWhat matters for a fish in transit is oxygen, temperature, darkness and minimal shaking. Sealing it in a bag of tank water with plenty of air secures oxygen, the foam box prevents rapid chilling from the 12°C outside air, and keeping it dark and still reduces stress. Tap water damages the gills with chlorine and is the wrong temperature. A bucket spills, and the jolting and light make the fish thrash and collide. Transport without water suffocates it within minutes. Tell the vet the collection site and date, tank size, number of fish, water temperature, water-change frequency, water test results and any medications used, and if possible bring tank water in a separate bag so the water can be checked too. After the trip, return the fish to its original container with same-temperature water and keep it dark and quiet.

IssuesHow to transport a fish had not been looked up in advance, and the three points of oxygen, temperature and shaking were not considered. The husbandry information to give the vet had not been organized.

ImprovementKeep transport bags, a foam box, ice packs and hand warmers on hand and learn the transport procedure. Keep a husbandry log (temperature, water-change dates, water tests, collection date) in a notebook so it can be shown at the clinic right away. Post the contact details of a vet who treats fish near the tank.

August, 1 a.m. Of 12 pale chub and bitterlings in a 60 cm tank, 5 are on their sides and the rest are gasping at the surface. The water is 29°C and the aeration is running. A water test shows high ammonia. A large amount of food was given in the evening and plenty is left on the bottom. The owner knows of no fish vet to call at night.

What is the most appropriate first action?

1. Wait until morning and consult the vet when the clinic opens
2. Remove and dispose of the 5 fish on their sides to protect the rest
3. Add fish medication to the tank to suppress bacterial infection
4. Change half the water with same-temperature dechlorinated water, siphon out the leftover food, turn the aeration to maximum, and support the fish on their sides in a separate container facing the current

AnswerD. Change half the water with same-temperature dechlorinated water, siphon out the leftover food, turn the aeration to maximum, and support the fish on their sides in a separate container facing the current

ExplanationSeveral fish collapsing at once points to poisoning or oxygen depletion from a sudden change in water quality; here the cause is rotting leftovers from the evening's heavy feeding and rising ammonia. Even without access to a vet, the owner can remove the cause on the spot. Change half the water with same-temperature dechlorinated water to lower ammonia, siphon out the leftovers, and turn the aeration to maximum to secure oxygen. Place the fish on their sides in a separate container near the aeration and support them with their mouths facing the current. Waiting until morning means the rest collapse too. Fish on their sides with moving gills can recover, so disposal is premature. Fish medication does nothing for poisoning and only adds stress. After dawn, consult a vet who treats fish about any fish that have not improved. When several fish collapse at once, the rule is to suspect the water, not the fish.

IssuesA large amount of food was given in hot summer water and the leftovers were left in place. The owner did not know the principle of suspecting the water when several fish collapse at once, and had not identified anyone to consult at night.

ImprovementFeed 1–2 times a day, only what is eaten in 2 minutes, and even less in summer. Keep stored dechlorinated water and test strips on hand so water can be changed even at night. Look up a vet who treats fish and an after-hours contact in advance.

A June night. A bitterling that jumped out of the tank and was found on the floor has been returned to water, but its gills barely move and it stays on its side. A family member says, "We should press its chest and do CPR like you would for a dog," and is about to pick up the fish and press its belly. The gills move faintly once every few seconds.

What is the most appropriate action?

1. Do not do chest compressions; in the water, support it near the aeration with its mouth facing the current so water flows over the gills, and wait
2. Do as the family member says and press the belly with your fingers for CPR
3. Blow air into its mouth as artificial respiration
4. Take it out of the water and warm its body to try to revive it

AnswerA. Do not do chest compressions; in the water, support it near the aeration with its mouth facing the current so water flows over the gills, and wait

ExplanationA fish's body is small and its organs are arranged differently from a person's or a dog's; pressing the belly with fingers damages organs, scales and slime coat and is fatal. Fish resuscitation comes down to one thing: passing oxygenated water over the gills. In the water, support it near the aeration with its mouth facing the current so water passes over the gills, and wait until it moves on its own. If the gills move even faintly, recovery is possible. Blowing air into the mouth does nothing since fish cannot breathe air, and it only damages the gills. Taking it out of the water to warm it dries the slime coat, the gills stop working, and it suffocates. If the gills stop after several hours, judge it dead. On recovery, add a slime-coat protectant, keep it dark and quiet, and observe for several days. If it is still on its side the next day, consult a vet who treats fish.

IssuesThe family tried to apply dog-and-cat CPR knowledge directly to a fish, not knowing the life-saving measures suited to fish. The procedure for a jumped-out fish had not been shared in advance.

ImprovementShare with the family that fish rescue follows the principle: in water, current, gills, dark and quiet. Keep an air pump and isolation container on hand and post the jump-out procedure near the tank. Close lid gaps to prevent jumping in the first place.