Koi / Nishikigoi

Koi herpesvirus disease (KHV)

SignsA run of sudden deaths in spring or fall at 18–28°C, gills rot and turn white, eyes become sunken

PreventionQuarantine new fish in a separate container for 3–4 weeks. On detection, report to the prefectural government and never release the fish

Hole-in-the-side disease (ulcer disease)

SignsScales on the flank redden, swell and fall off; a hole opens in the skin exposing muscle

PreventionCommon in spring and fall at 15–20°C. Keep water quality up with water changes and avoid causing wounds

White spot disease (ich)

SignsMany white spots on the body and fins; the fish rubs itself against the bottom or walls

PreventionAvoid sudden drops in water temperature. Quarantine new fish and plants before introducing them

Anchor worm / fish lice

SignsThread-like or disc-shaped parasites attached to the body, flashing (rubbing) behavior and red hemorrhagic spots

PreventionMainly brought in with purchased fish or plants. Quarantine, and clean fallen leaves and mud from the pond

Dropsy (pinecone disease)

SignsScales stand out like a pinecone, the belly swells and the fish becomes sluggish

PreventionCaused by poor water quality and overcrowding. Do regular water changes, keep stocking density appropriate and do not feed old food

Gill disease (gill rot)

SignsRapid gill-cover movement or only one side closes, gasping at the surface, gills darken

PreventionPrevent buildup of ammonia and nitrite. Clean the filter and manage stocking density

Jumping out / falls

PreventionKeep at least 20 cm between the pond or tank rim and the water surface, and use a net or lid. Do not startle the fish

First aidLift gently with wet hands or a wet towel and return to water. A dried-out mucus layer is fatal; rest in a 0.5% salt bath

Abrasions on the body

PreventionRemove sharp edges on stones and pipes in the pond; use rubber nets. Guide the fish into a bag underwater when catching

First aid0.5% salt bath to prevent infection; if the wound swells white or spreads red, see a vet within a few days

Split fins / fin bleeding

PreventionPrevent jumping out and overcrowding. Reduce density during the chasing (spawning) season

First aidHeals naturally with rest in clean water. If clouding or rotting progresses, it is a bacterial infection: see a vet

Sunburn / sores on the back

PreventionDo not expose shallow ponds or tanks to direct sunlight. Shade with cover or floating plants

First aidProvide shade and a 0.5% salt bath. If the sores spread or swell red, see a vet

Nontuberculous mycobacterial infection

RouteThrough cuts on the hands via pond water or the fish's body surface

PreventionDo not touch the water if you have cuts on your hands. Wash hands with soap after work

Aeromonas infection

RoutePond water or fish mucus entering a wound or the mouth

PreventionDo not drink pond water. Dress wounds before working and wash hands thoroughly

Salmonellosis

RouteOral infection from hands that touched pond water or mud

PreventionWash hands after work. Children and the elderly should sanitize hands after pond work

Leptospirosis

RouteSkin infection from pond water contaminated with rat urine

PreventionControl rats around the pond. Never enter the pond barefoot; use boots and gloves

Food

• Koi pellets 2–3 times a day at 20–28°C, as much as they finish in 5 minutes
• Reduce to once a day below 15°C; stop feeding below 10°C
• Color-enhancing or high-protein food in summer only; easily digested food in spring and fall

Water

• Always dechlorinate tap water; change one third of the water weekly as a guide
• Keep pH around 7, ammonia 0 and nitrite 0; test at least monthly

Toys & play

• Hand-feed to tame them (toys are not needed)
• Use a pump to create current and stimulate swimming
• Vary the food by season to encourage foraging behavior

Nest, bed & hideout

• Make a deep area of 1 m or more as a refuge in summer and winter
• Hiding places against herons (shade boards, pipes, bridges)

Bedding & litter

• Keep the pond bottom bare or concrete; mud breeds pathogens
• In tanks use no substrate or only a very thin layer for easy cleaning
• Siphon out fallen leaves and sludge regularly

Cage / housing material

Concrete, FRP or liner pond, or a large acrylic tank

Size & space

Pond: at least 1 ton of water per fish; tank: 90 cm or larger, for young fish only

Layout

• Install a filter with a capacity of at least one third of the pond volume
• Create a deep area of 1 m or more and a shallow feeding spot
• Supply oxygen with a waterfall or fountain and circulate the water

Placement notes

• Avoid spots in direct sunlight all day; provide shade
• Choose a site where rainwater, pesticides or muddy runoff cannot flow in during heavy rain

Exercise, free-roam & walks

• No walks needed; keep an anti-jump net in place at all times
• Fish kept in tanks must be moved to a pond or large tank as they grow
• Limit catching and moving to a few times a year to reduce stress

Temperature & humidity

TemperatureIdeal 18–28°C. Keep changes within 2°C per day

HumidityMaintain dissolved oxygen at 6 mg/L or more (humidity control not needed)

Summer & winter

• Summer: above 30°C, add shade, boost aeration and reduce feeding
• Winter: fast through the season; in freezing regions ensure a deep area and a hole in the ice

Nail trimming

No nail trimming. Instead, test water quality and clean the filter monthly

Brushing, skin & coat care

Do not touch the body; it damages the mucus layer. Inspect the body surface underwater at feeding time

Other care

Have anchor worms removed with tweezers at a fish vet. Do not add medication on your own judgment

Protection from predators

• Herons and cormorants: stretch nets or fishing line over the pond
• Raccoons and cats: cover the pond with a net at night
• Rats: seal food bags around the pond to prevent intrusion

Foreign-body ingestion

Their bottom-poking habit makes them suck in pebbles or plastic scraps. Keep small objects that fit in the mouth out of the pond and do not drop torn pieces of food bags

Being stepped on

When the pond is drained for cleaning, do not step on fish left on the bottom. Check your footing and move the fish to a separate container first

Getting pinched

Fins or the body can be sucked into pump intakes or drains. Always fit a guard or net on the intake

Falls

Falls from jumping out are the most common. Keep 20 cm or more between the surface and the rim and install a net. If one falls, return it with wet hands

Escape

Fish can wash out of the pond when heavy rain raises the level. Fit a screen to the overflow pipe and lower the water level before a typhoon. Escape into a river counts as illegal release

Safe release procedure

1. Being sucked on during hand-feeding does not hurt; just relax and it will let go
2. If a large fish sucks your hand, hold still and wait for it to release
3. Remove rings beforehand and keep bare cuts out of the water
4. If you must lift the fish, support the belly with wet hands and handle it underwater

After the release

Wash hands with soap. If a cut that touched the mucus swells red, suspect mycobacterial infection and see a dermatologist

What to tell the vet & what to bring

Water temperature, pH, ammonia and nitrite readings; photos of the pond water and the fish; history of new fish, water changes and medications

Checking consciousness, breathing & heartbeat

• Gill-cover movement (normal is 40–80 per minute, varies with temperature)
• Does it flee when approached, do the eyes move, can it right itself
• Even on its side, if the gills are moving there is a chance of survival

Stopping bleeding

• Bleeding from fins or the body surface stops naturally in clean water
• Improve pond water quality and prevent secondary infection with a 0.5% salt bath
• If bleeding does not stop or spreads, go to a fish vet

Transport

• Pond water and oxygen in a bag or a large sealed container, placed in a foam box
• Keep it dark, maintain water temperature, avoid jolting and travel in the shortest time possible