Koi carp
A large ornamental carp bred in Japan from an East Asian food fish, reaching sixty centimetres or more and living for decades. It is a pond animal rather than an aquarium one, and every aquarium sale of a koi is a sale of a fish that will outgrow the tank.
Quick profile
- Scientific name
- Cyprinus rubrofuscus
- Adult size
- 45–75 cm (17.7–29.5 in)
- Lifespan
- 25–50 years
- Temperature
- 4–28 °C (39–82 °F)
- pH
- 7–8.5
- Hardness
- 8–25 °dGH
- Minimum tank
- 5000 L (1321 US gal)
- Minimum length
- 400 cm
- Swimming zone
- All levels
- Social
- Shoaling — at least 3
- Diet
- Omnivore
- Overall confidence
- moderate confidence
What keeping this species involves
Undemanding in temperament and enormous in every practical requirement: volume, filtration, winter depth and a commitment that can outlast the person who started it.
Slow, warm, often turbid lowland rivers, lakes and floodplains with soft bottoms and heavy vegetation.
Identity and names
- Common name
- Koi carp
- Scientific name
- Cyprinus rubrofuscus
- Family
- Cyprinidae
- Order
- Cypriniformes
- Class
- Actinopterygii
- Also sold as
- Cyprinus rubrofuscus, koi, nishikigoi, Amur carp, Cyprinus carpio haematopterus
- Former or synonymous names
- Cyprinus carpio haematopterus
Name ambiguity
Ornamental koi derive from the East Asian carp, treated by current work as Cyprinus rubrofuscus rather than as a variety of the European Cyprinus carpio. Older aquarium and pond literature files them under carpio and the two names are both searchable.
Adult size and growth
45–75 cm (17.7–29.5 in) — Figures for well-kept pond koi. Fish kept in a tank grow more slowly and do not stop growing, which is a welfare problem rather than a size solution.
Typical lifespan in an aquarium: 25–50 years. Almost every animal sold in a shop is a juvenile, so the tank has to suit the size in the first line rather than the size in the bag.
Shape, which is what the compatibility engine reads instead of length alone: fusiform — the ordinary torpedo with a small terminal mouth. Assigned from what is typical of Cyprinidae, not examined for this species.
Water and tank requirements
The published minimum is a small pond expressed in aquarium terms, because there is no aquarium answer. A fish reaching well over half a metre and living for decades needs a body of water rather than a tank.
Temperature (°C)
Scale: 0 to 36 °C.
pH (pH)
Scale: 4 to 9.5 pH.
Hardness (°dGH)
Scale: 0 to 30 °dGH.
Why these ranges: A temperate species that overwinters at near-freezing temperatures and feeds actively in summer warmth. The band is the annual range of a temperate pond rather than a target to hold steady, and the seasonal cycle is part of normal husbandry.
Where these figures come from
Habitat class
No wild referent
An animal bred in captivity for long enough that its wild habitat no longer describes the stock in the shops.
The published band describes farmed stock, not a river. That is the honest thing for it to describe, and it means wild-habitat figures for the species are interesting history rather than husbandry.
These are figures for the KIND of water body, not measurements of this species. They are AquariumHQ’s summary of the general limnology and are given wide because the water is.
- TemperatureDescribes farmed stock, not a wild habitat
- This animal is not sold as a wild-caught fish and has not been for a very long time. The band describes the water commercial stock is bred and raised in, which is the useful thing for it to describe; the wild population's water is a separate question and is not what a keeper is buying.
- pHDescribes farmed stock, not a wild habitat
- This animal is not sold as a wild-caught fish and has not been for a very long time. The band describes the water commercial stock is bred and raised in, which is the useful thing for it to describe; the wild population's water is a separate question and is not what a keeper is buying.
- General hardnessDescribes farmed stock, not a wild habitat
- This animal is not sold as a wild-caught fish and has not been for a very long time. The band describes the water commercial stock is bred and raised in, which is the useful thing for it to describe; the wild population's water is a separate question and is not what a keeper is buying.
| Requirement | Value | Notes |
|---|---|---|
| Temperature | 4–28 °C (39–82 °F) | Stability matters more than hitting an exact figure. |
| pH | 7–8.5 | A stable pH outside the ideal band is usually better than an unstable one inside it. |
| General hardness | 8–25 °dGH | Check your supplier's published figure before choosing species. |
| Minimum volume | 5000 L (1321 US gal) | A floor, not a recommendation. |
| Minimum footprint | 400 cm long × 200 cm deep | The published minimum is a small pond expressed in aquarium terms, because there is no aquarium answer. A fish reaching well over half a metre and living for decades needs a body of water rather than a tank. |
| Water flow | moderate | Match the outflow to this rather than to the filter's rating. |
| Lid required | Yes | This species jumps. |
| Structure | Open water with substantial filtration and depth enough to overwinter. Planting will be eaten or uprooted. |
Behaviour
- Roots through the bottom for invertebrates, which stirs up sediment continuously and is the main reason koi ponds need the filtration they do.
- Learns to take food by hand and recognises individual people at the pond edge, which is well documented and part of why the fish is kept.
Telling males from females
Males are slimmer and develop breeding tubercles on the gill covers and pectoral fins in spring. Females are deeper-bodied. Neither is readable in young fish.
Feeding
Feeding is temperature-dependent: digestion effectively stops in cold water, and feeding a koi below about ten degrees Celsius does harm rather than good.
- Diet type
- Omnivore
- Staples
- Floating koi pellets, Blanched lettuce, Peas, Prawn
Breeding
- Strategy
- Eggs scattered over vegetation in spring, with no parental care
- Difficulty
- straightforward
- Parental care
- none
Adults eat the eggs and fry readily. Ornamental colour does not breed true, and most offspring of two show koi are plain.
Tankmates and compatibility
Assessments are derived from stored traits by the compatibility engine, with the reasoning shown. A verdict is a statement about probability, not a guarantee — individual temperament can override it.
| Towards | Risk | What it means |
|---|---|---|
| Small fish | moderate | Anything that fits in the mouth is at risk once this species reaches adult size. |
| Adult dwarf shrimp | near-certain | Cover reduces losses but rarely prevents them where the risk is high. |
| Shrimplets | near-certain | Determines whether a shrimp colony can grow rather than merely persist. |
| Snails | high | Applies to snails you wanted as well as snails you did not. |
| Long or trailing fins | none | Fin damage is an entry point for secondary infection. |
| Plants | uproots | Tough species such as Anubias and Java fern survive most grazers. |
Assessed pairings
Any pair in the catalogue can be assessed with the compatibility checker.
Welfare considerations
Points where the way this species is commonly sold or kept differs from what it actually needs.
- Sold in aquarium shops at eight centimetres alongside tropical fish. Nothing about an indoor tank suits this animal and the practice is a persistent welfare problem.
- Common carp are among the most damaging introduced freshwater fish worldwide, and ornamental koi are the same biological species as the East Asian carp. Release into open water is prohibited in many jurisdictions and is destructive everywhere.
Problems to watch for
Most problems in a tank containing this species are general rather than species-specific. Start with the troubleshooting index and work from the observation rather than from a guess at the cause.
What AquariumHQ does not yet hold
Species-level disease susceptibility is modelled in the graph and is not yet populated. Health pages will not be published until each condition has veterinary sourcing, because health content written without evidence is the most harmful thing this product could publish. The gap is recorded in the coverage report.
Where it comes from
- Native range
- East Asia — the Amur, Yangtze and Pearl river systems, with the ornamental form worldwide
- Habitat
- Slow, warm, often turbid lowland rivers, lakes and floodplains with soft bottoms and heavy vegetation.
What the checklists say
9 distribution statements retrieved from GBIF checklists. Establishment means is reproduced from the source where it states one, and left unstated where it does not — a record of presence is not a record of origin.
| Status | Places | What this means |
|---|---|---|
| Introduced | 5 | A checklist states the species arrived here with human help. That is not a statement that it is established, harmful or illegal. |
| Recorded as present, establishment means not stated | 4 | The source records the species as present and does not say whether it is native or introduced. AquariumHQ does not fill that in. |
No checklist AquariumHQ holds positively calls this species native anywhere, so the native range above rests on AquariumHQ’s own reading of the literature rather than on a cited checklist.
Legal status and responsible practice
Restriction note
Carp introductions have caused severe ecological damage in many regions, and rules on keeping, moving and releasing them differ sharply between jurisdictions — some require permits for ponds, others prohibit stocking entirely. Check the specific national and local rules; the position for ornamental koi is frequently distinct from that for wild carp.
Never release aquarium life into the wild
Aquarium animals, plants, and aquarium water must never be released into ponds, rivers, canals, drains or the sea — not even in a country where the species is sold freely. Released fish and invertebrates establish populations that displace native species, and released plants and water carry pathogens, parasites and fragments capable of spreading without any animal at all. If you can no longer keep an animal, rehome it through an aquarium society, a retailer that accepts returns, or another keeper.
- Rehome through a local aquarium society, a specialist retailer or another keeper.
- Unwanted plants go in household waste or a hot compost heap, never in a watercourse or a garden pond.
- Never empty a bucket of tank water into a drain that discharges to open water.
- If an animal is beyond rehoming, ask an aquatic veterinarian about humane euthanasia rather than releasing it.
Legal status is recorded from the sources named and was correct on the verification date shown. Rules change, and sub-national rules often differ from national ones. Check the current position with the relevant authority before acquiring, selling or moving any animal or plant.
Evidence
Every stored value with the kind of support behind it and where to check it. Evidence type and confidence are separate axes: an editorial position held confidently and a measurement held tentatively are different things.
| Claim | Value | Evidence type | Confidence | Source |
|---|---|---|---|---|
| scientific name | Cyprinus rubrofuscus | Observed / measured | high confidence | Catalogue of Life; GBIF — Global Biodiversity Information Facility |
| native range | East Asia — the Amur, Yangtze and Pearl river systems, with the ornamental form worldwide | Observed / measured | moderate confidence | GBIF — Global Biodiversity Information Facility |
| adult size | 45–75 cm | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| lifespan | 25–50 years | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| temperature | 4–28 °C | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| ph | 7–8.5 pH | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| general hardness | 8–25 °dGH | Aquarium-literature consensus | moderate confidence | AquariumHQ husbandry synthesis |
| minimum tank volume | ≥ 5000 L | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
| minimum tank length | ≥ 400 cm | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
| minimum group size | ≥ 3 | AquariumHQ editorial position | moderate confidence | AquariumHQ husbandry synthesis |
Last reviewed 2026-08-10. Values labelled as an aquarium-literature consensus are AquariumHQ editorial positions rather than measurements — see how evidence is handled.
Why we say this
What each figure on this page rests on, and how far that is from being independently established.
Every figure on this page has a provenance. Open any claim to see what AquariumHQ holds for it, which sources say what, and what a claim of that kind actually needs. Different claims need different evidence: an accepted scientific name is settled by one authority, while a legal restriction needs a current primary instrument. A single source is often the right answer rather than a thin one.
scientific nameAquariumHQ's own position, externally checkableBased on a verified authoritative source.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: accepted scientific name and classification — 1 authoritative source in an appropriate context. Checked against gbif on 2026-08-29. One authoritative backbone is what this class requires.
What would improve this: Go and read a source. This is reachable and has not been reached.
native rangeAquariumHQ's own position, externally checkableBased on a verified authoritative source.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: where the species occurs — 1 authoritative source in an appropriate context. 9 distribution statements held from GBIF checklists — introduced in 5 country. Establishment means is reproduced from the source, never inferred from occurrence.
What would improve this: Go and read a source. This is reachable and has not been reached.
adult sizeOne sourceAquariumHQ's own position, derived from evidenced inputs.1 citation, 1 independent origin
A single source: Page and Burr 1991.
- 122 cm — USGS Nonindigenous Aquatic Species database · Maximum recorded · Observed in the wild · checked 2026-08-29
What this kind of claim needs: size a well-kept specimen reaches — no external source is required, provided AquariumHQ shows its reasoning. A documented maximum is held and the catalogue's typical adult range is stated separately, with the relationship between them recorded rather than collapsed. A well-grown adult may be substantially larger than the typical figure — 122 cm TL against a typical 45–75 cm. Large enough to change what tank is appropriate.
What would improve this: Corroborate against a second, independent source.
lifespanAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
temperatureAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
phAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
general hardnessAquariumHQ's own position, externally checkableAquariumHQ's own position, derived from evidenced inputs.
AquariumHQ's own position on a claim an external source could support.
What this kind of claim needs: what aquariumhq advises for keeping — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Go and read a source. This is reachable and has not been reached.
minimum tank volumeAquariumHQ's judgement — no authority publishes thisAquariumHQ's own position, derived from evidenced inputs.
No external authority publishes this. The rationale carries the reasoning.
What this kind of claim needs: minimum tank size — no external source is required, provided AquariumHQ shows its reasoning. No authority publishes this. AquariumHQ derives it, and the derivation is recorded with its inputs and assumptions.
What would improve this: Keep the rationale current. There is no source to find.
minimum tank lengthAquariumHQ's judgement — no authority publishes thisAquariumHQ's own position, derived from evidenced inputs.
No external authority publishes this. The rationale carries the reasoning.
What this kind of claim needs: minimum tank size — no external source is required, provided AquariumHQ shows its reasoning. No authority publishes this. AquariumHQ derives it, and the derivation is recorded with its inputs and assumptions.
What would improve this: Keep the rationale current. There is no source to find.
minimum group sizeAquariumHQ's judgement — no authority publishes thisAquariumHQ's own position, derived from evidenced inputs.
No external authority publishes this. The rationale carries the reasoning.
What this kind of claim needs: group size — no external source is required, provided AquariumHQ shows its reasoning. No external source is required for this class. AquariumHQ's position stands on its stated reasoning.
What would improve this: Keep the rationale current. There is no source to find.
Guides relevant to this species
Chosen from this record's own traits rather than listed on every page.
- Schooling and shoalingThis species is recorded as shoal, and the number it is kept in decides how it behaves.
- Predator and prey sizeIt takes adult dwarf shrimp, which decides what can share the tank.
- Native, introduced and invasiveThis record carries an invasive or legal note, and those are five separate claims that are easy to read as one.
- Scientific and trade namesThis species has been published under more than one accepted name.
- Wild conditions and aquarium recommendationsThe water band on this page is an AquariumHQ position with a stated reason, not a measurement of a river.
- Why fish size figures disagreeThe largest recorded specimen is far enough above the typical adult here to change what tank is appropriate.
- Rehoming an animal you can no longer keepThis animal reaches 75 cm. Most of the difficult rehoming cases are fish sold small that were always going to become impossible.
Group size and social behaviour
A sociable fish that lives in groups. A single koi in a pond is a common arrangement and not a good one.