Cyprinus rubrofuscus (ornamental varieties)
Koi
Koi are ornamental carp that can exceed 60 cm and live for decades. They need a large, deep, well-filtered pond rather than an aquarium, plus seasonal feeding adjustments.
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At a glance
- Scientific name
- Cyprinus rubrofuscus (ornamental varieties)
- Family
- Cyprinidae
- Origin
- Selectively bred in Japan from Amur carp
- Lifespan
- 25–35 years, occasionally over 50
- Adult size
- 60–90 cm
- Weight
- 5–15 kg
- Temperament
- Tame, interactive pond fish that learns to feed from the hand.
- Activity level
- Moderate; graceful, steady swimmers.
- Social needs
- Gregarious; keep at least three together.
- Intelligence
- Modest but recognises keepers.
- Noise
- Silent.
Housing and environment
- Housing
- A pond of at least 4,500 litres and 1.2 m deep for a small group; filtration sized generously.
- Space requirement
- Garden pond only — never an aquarium long-term.
- Temperature
- Tolerates 2–30 °C with seasonal acclimatisation; needs an ice-free gas-exchange hole in winter.
- Humidity
- Not applicable.
- Exercise
- Needs a large, deep pond to swim and overwinter.
- Grooming
- Not applicable.
Life stages and changing needs
Expanded care guidance · Updated 7 October 2026 · Not professionally reviewed
Young animals and settling in
Young coldwater carp such as goldfish and koi grow continuously and are often sold far too small for the housing they will eventually need. Confirm the adult size of the exact variety before choosing a tank or pond, and cycle the system fully before any fish arrive. A tiny juvenile in a small aquarium or basic pond liner will outgrow that space within a year or two, not remain miniature; plan upgrade pathways in advance rather than reacting once welfare already is compromised.
Adult care and reproductive changes
Adults are heavy, long-lived waste producers that need generous volume, strong filtration and, for single-tailed and pond varieties, genuine swimming length rather than a decorative container. Spawning can occur among healthy group-kept fish without intervention; chasing during spawning is usually brief, but persistent harassment, torn fins or an exhausted fish needs separation. Unwanted fry should never be released into public waterways, where both goldfish and koi are considered invasive.
Later life and changing needs
These fish can live for decades with good water quality, so ageing is measured in years, not months, and a healthy older fish should still swim, feed and hold normal buoyancy. A fish that struggles to stay upright, floats persistently or sinks is more likely showing swim bladder or organ disease than simple old age. Maintain stable water parameters and seasonal feeding adjustments throughout life, and seek an aquatic vet's opinion rather than assuming slow decline is inevitable.
Diet and feeding
Pond pellets adjusted seasonally; wheatgerm-based food in cool water.
Offer a quality pellet formulated for coldwater fish of this type, supplemented with blanched vegetables and occasional appropriate frozen foods; the species notes on this page explain whether floating or sinking food suits it best. Reduce or stop feeding when water temperature drops below the point at which the fish's metabolism slows, typically in winter for pond fish, since food can sit undigested in a sluggish gut and foul the water.
Feed small amounts that are cleared within a few minutes rather than filling the fish to capacity, and remove uneaten food promptly. Overfeeding is the most common cause of poor water quality in goldfish and koi systems. A fish that stops eating should prompt an immediate water test for ammonia, nitrite and temperature before any other explanation is assumed.
Daily care and ongoing checks
Every day
Check filtration, water flow, temperature and the behaviour of every fish each day; in ponds, confirm that an area stays free of ice in winter so gases can exchange even though the fish are barely feeding. Watch for fish isolating themselves, gasping at the surface or showing damaged fins or scales, since early detection of poor water quality or disease prevents larger losses.
Regular maintenance and prevention
Base partial water changes on test results and stocking density rather than a single fixed rule, and always treat replacement water for chlorine or chloramine. Protect ponds from herons, cats and other predators with netting or adequate depth, and quarantine every new fish separately for several weeks before introduction, since infections such as koi herpesvirus in koi or cyprinid herpesvirus 2 in goldfish can devastate an established collection.
Behaviour and compatibility
Long-lived commitment measured in decades; herons and cats are real predators.
- With children
- Wonderful family pond fish with supervision around water.
- With other animals
- Keep with other koi or large goldfish; protect from predators.
- Training difficulty
- Learns feeding routines readily.
Health
- Koi herpesvirus from unquarantined new fish
- Parasites (flukes, anchor worm)
- Ulcers in poor water
- Winter stress in shallow ponds
Quarantine every new koi for at least four weeks; KHV is incurable and devastating.
Special care requirements
See the housing, diet and health sections — this species has needs that are easy to underestimate.
Pond scale, winter care and quarantine
Koi are usually fed floating pond pellets so keepers can watch appetite and inspect each fish at the surface; feed only what is eaten within a few minutes.
Koi are not aquarium fish; they need a pond of at least 4,500 litres and 1.2 metres deep for even a small group, sized generously because these fish can reach 60 to 90 centimetres and live for decades. An undersized or shallow pond that seems adequate for young koi will become a serious welfare problem within a few years as they grow.
Maintain an ice-free area of open water through winter so gases can still exchange even when fish are barely feeding in the cold, and quarantine every new koi for at least four weeks before introducing it to an established pond, since koi herpesvirus is incurable and can wipe out a whole collection if introduced without quarantine.
Before you take one on
Sources and further guidance
These welfare and veterinary resources inform general husbandry advice; they do not certify this profile or replace a species-experienced professional. Confirm exact environmental settings and feeding plans for your animal.
Can your home meet koi needs?
- Plan for a lifespan of 25–35 years, occasionally over 50, including holiday cover and, where one is available, a vet familiar with this species. Lifespan ranges are not guarantees.
- Arrange the social setting before arrival. Social guidance for this species: Gregarious; keep at least three together. Do not assume another animal of any species is a suitable companion.
- Habitat priority for this species: Garden pond only — never an aquarium long-term. Make sure your setup can provide this consistently. Check temperature and water quality with a thermometer and test kits rather than guessing.
- Key care point: See the housing, diet and health sections — this species has needs that are easy to underestimate. Discuss diet and any specialist needs with the supplier, rescue and a vet who treats this species, and check local legal requirements before acquiring an animal.
Educational reference, not an individual diagnosis or a verified professional review. Read our editorial policy.
Guidance libraries and further reading
These organisations publish relevant guidance. Their links are further reading, not evidence that they have reviewed this page.