Koi
Also known as Nishikigoi, Japanese koi
- Adult size
- Commonly 45–60 cm; some reach 90 cm or more
- Lifespan
- 25–35 years
- Origin
- Japan, bred from carp, particularly in the Niigata region
- Company
- Keep in a group in a large pond
- Tame
- Peaceful
- Large
- Long-lived
About the Koi
Koi are ornamental varieties of carp, bred in Japan for their striking patterns of red, white, black, yellow and metallic tones. Varieties have names such as kohaku, sanke and showa, and serious enthusiasts judge them on colour, pattern and body shape.
Koi are large, powerful fish that are kept almost exclusively in ponds. They can grow very big and are known for their long lives, often outliving the people who first bought them.
Over time koi become remarkably tame, and many will take food from the hand. Their size and long lifespan mean that keeping them is a serious, long-term commitment to a well-built pond.
Temperament
Koi are peaceful, sociable fish that should be kept in groups. They are not aggressive but will eat small fish, fry and many pond plants. They can share a large pond with goldfish, though they will outgrow them considerably. Many become confident and interactive, gathering wherever their keeper stands.
Tank & water
Koi need a large, deep, purpose-built pond; several thousand litres is a sensible starting point, with depth of over a metre often recommended so fish can escape temperature extremes and predators. Strong mechanical and biological filtration, plus good aeration, are essential because koi produce a lot of waste. A new pond must be fully cycled before fish are added. Netting or other protection from herons is wise.
Diet
Koi are omnivores and do well on a good-quality koi pellet, with feeding adjusted to water temperature. In warm months they eat readily, while in cold weather their digestion slows sharply and feeding should be reduced or stopped. Fruit, lettuce and other treats can be offered occasionally, but uneaten food should not be left to rot in the pond.
Behaviour & tank mates
Koi enjoy plenty of swimming space and interact readily with their owners, especially at feeding time. Plants on the margins or in protected areas offer shade and forage, though koi will uproot and eat many plants. Keeping them in a group lets them show their natural social behaviour.
Tank maintenance
Pond maintenance means regular filter cleaning (using pond water to protect the bacteria), partial water changes with dechlorinated water and removing debris and leaves. Test water regularly, particularly in spring and in hot weather.
Health
Koi are susceptible to external parasites, ulcers and bacterial infections, especially when water quality is poor or in early spring. Koi herpesvirus is a serious viral disease that can cause rapid deaths, and in some countries it must be reported to the authorities. Quarantine all new koi for several weeks, buy from reputable sources and contact an aquatic vet promptly if fish die or show sudden illness.
Prone to
- External parasites
- Ulcers
- Bacterial infections
- Koi herpesvirus
- Fin rot
Call a vet if you notice
- Fish gasping at the surface or crowding near water inlets — test the water and increase aerationEmergency
- Sudden deaths in the pond
- Open sores or ulcers
- Pale, damaged or mottled gills
- Flashing or rubbing against the sides
- Lethargy and refusal to feed in warm water
An aquatic vet or experienced aquatic specialist is best placed to help. Find one before you need them.
Common questions
Can koi live in an aquarium?
Only temporarily, such as during quarantine. Koi grow far too large for home aquariums and need a big, deep pond.
How long do koi live?
Koi commonly live for several decades in a well-kept pond. Some individuals are reported to have lived much longer, though claims of extreme ages are hard to verify.
Can koi and goldfish live together?
Yes, they can share a large pond, as both are coldwater fish with similar needs. Bear in mind koi will grow much larger, and the two can interbreed.
Should I feed koi in winter?
When the water is very cold, koi digest food poorly, so feeding is usually reduced or stopped. As the water warms in spring, feeding is gradually increased.
Written by the Petopedia editorial team to our editorial standards. General guidance, not veterinary advice.