What tank size does a Koi need?

A Koi needs an aquarium of at least 4000 litres (1056.7 US gallons) with a footprint at least 200 cm (78.7 in) long — for this species, swimming length matters as much as raw volume.

What water parameters does a Koi need?

Koi does best at 4–28 °C (39.2–82.4 °F), pH 6.5–8.5 and a general hardness of 4–20 °dGH.

What group size does a Koi need?

Koi is a social species — keep a group of at least 5 so natural behaviour emerges and stress stays low.

Is the Koi suitable for beginners?

Koi sits in the middle of the difficulty scale — manageable for a careful beginner who has done the reading, but it has specific needs worth researching first.

Species library

Koi

Cyprinus carpio

Cyprinidae

FishDifficulty: ●●●○○Data: well-established

📷 Bramans · CC BY-SA 4.0 · resized · Wikimedia Commons

Koi — Cyprinus carpio — 📷 Bramans · CC BY-SA 4.0 · resized · Wikimedia Commons

About

The koi is the ornamental form of the common carp, refined in Japan and prized for bold red, white, orange and black patterning over a heavy, barbeled carp body. It is a true pond fish: adults reach 60-90 cm, live for several decades, and cruise all levels in sociable groups while rooting through the bottom for food, uprooting plants and stirring sediment as they go. It quickly outgrows any indoor aquarium.

Care

Pond-scale fish only: budget at least 1000 L per adult with powerful filtration and aeration. Thrives at 15-25 °C, tolerates roughly 4-30 °C and overwinters outdoors under ice, but its coldwater nature and enormous bioload make it incompatible with tropical community tanks. Jumps when startled, so cover or deep freeboard is needed.

Diet

Feed a staple koi pellet plus vegetables, insect larvae and crustaceans; reduce feeding in cool water and stop below about 10 °C.

Tankmate compatibility

Avoid (271)

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Computed by the AquaSage engine for a spacious reference tank — always check the full verdict for your exact setup in the calculator.

Telling males from females

What works for this species, most reliable first.

Breeding tuberclesStrong · only in breeding condition — they vanish out of season
Goldfish and their cyprinid relatives grow small white bumps on the gill covers and leading pectoral rays when they come into breeding condition.

All the methods, with diagrams

Frequently asked questions

What tank size does a Koi need?: A Koi needs an aquarium of at least 4000 litres (1056.7 US gallons) with a footprint at least 200 cm (78.7 in) long — for this species, swimming length matters as much as raw volume.
What water parameters does a Koi need?: Koi does best at 4–28 °C (39.2–82.4 °F), pH 6.5–8.5 and a general hardness of 4–20 °dGH.
What group size does a Koi need?: Koi is a social species — keep a group of at least 5 so natural behaviour emerges and stress stays low.
Is the Koi suitable for beginners?: Koi sits in the middle of the difficulty scale — manageable for a careful beginner who has done the reading, but it has specific needs worth researching first.

Sources

Water & tank

Temperature4–28°C

Optimal temp.15–22°C

pH6.5–8.5

Hardness (GH)4–20°d

Carbonate (KH)3–12°d

Adult size90 cm

Min. volume≥ 4,000 L

Min. tank length≥ 200 cm

Group size≥ 5

Swimming zoneAll levels

TemperamentPeaceful

Water typeFreshwater

FlowLow

Lifespan~30 yrs

Try in the calculator