Koi

Koi are domesticated colour forms of the Amur carp (Cyprinus rubrofuscus), a hardy river and pond fish of East Asia developed over centuries of ornamental breeding in Japan and China and now kept in garden ponds worldwide. They are pond fish rather than tropical aquarium fish: hardy across a wide temperature swing, active in summer warmth and slowing to near dormancy under winter ice, and long-lived - commonly 25 to 50 years with reports of more.

Adults commonly reach 60 cm, with PetMD listing up to 3 feet (about 90 cm) and AquariumLab expecting 24 to 36 inches (60-91 cm) - so plan every enclosure for the record's 90 cm figure from day one. Koi grow fast while young and live 25 to 50 years, so undersized systems fail quickly. Pair the adult size with the per-fish volume rule (about 250 gallons/950 L per large adult in a group) when stocking. Confidence note: medium-confidence guidance drawn from published species care information; treat as conservative guidance, not a precise requirement.

Koi are peaceful, social pond fish that live in schools, learn to recognise their keepers and will take food from the hand. They root through soft substrate, uproot plants and swallow anything small enough. They cope with a very wide temperature swing - active in summer, near-dormant under winter ice - and when they jump it is a stress response (poor water quality, predators, spawning chases), not play, so repeated jumping means test the water first. Confidence note: medium-confidence guidance drawn from published species care information; treat as conservative guidance, not a precise requirement.

Koi are pond fish: one koi needs roughly 1,000 L to start, and a group needs a pond of several thousand litres with strong filtration (filter turning the volume over at least twice an hour). Run it cycled - never add koi to an uncycled tank - with slow water circulation and aeration that is not optional. Cover the water: koi jump 60-90 cm under stress, so fit netting or a secure lid with an air gap for gas exchange, and keep the temperature in the 15-25 C comfort band.

Read the 1,000 L minimum as floor area first: koi swim side to side, so a long, wide, moderately deep enclosure (on the order of 2.4 x 1 m of floor) beats a tall column, because a 60-90 cm fish turns at the ends of a straight lane. Keep the lane clear of hardscape, judge stocking by surface area rather than volume, and remember a narrow tank can hit the litre target while leaving the fish nowhere to turn. Confidence note: medium-confidence guidance drawn from published species care information; treat as conservative guidance, not a precise requirement.

Omnivore: feed a varied, koi-formulated diet - sinking pellets with greens and occasional live or frozen foods (TankBud). Give small portions eaten within a couple of minutes and scale to temperature: below about 13 C feed every few days, once daily around 13-21 C, twice daily above 21 C (PetMD), and withdraw food entirely at around 10 C when metabolism shuts down (K.O.I.). Remove leftovers daily; uneaten food fouls the water.

Keep koi with other large, robust pond fish only: goldfish (single-tailed varieties preferred), other koi, or cool-tolerant community fish. Anything small enough to swallow is food, plants get uprooted and small invertebrates are eaten. Do not overcrowd - overcrowded ponds lead to stress and disease (PetMD) - and treat 'at least five' as the social minimum, not a stocking target: volume and filtration set the real ceiling.

Verdict: a 1,000-litre-plus pond commitment, not a beginner aquarium fish. Koi produce heavy waste, so budget for powerful filtration, weekly water changes, mandatory aeration and a fully cycled system before any fish arrives. Cover the water because they jump under stress, keep them in a school of at least five with room to cruise a straight lane, and feed a varied formulated diet scaled to temperature. The reward is a social, hand-taming, 60-90 cm fish that lives 25-50 years; the cost is decades of filtration, water changes and pond-scale maintenance - PetMD rates the care 'Intermediate to advanced' even though this record says Beginner. Confidence note: medium-confidence guidance drawn from published species care information; treat as conservative guidance, not a precise requirement.