Koi are ornamental color morphs of the Amur carp, bred in Japan for over two centuries into hundreds of patterned varieties. They are large, long-lived, intelligent pond fish that grow far too big for typical aquariums and are best kept in spacious, heavily filtered outdoor ponds where their top-down colors and personalities can be fully appreciated.
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Commonly 24-36 in (60-90 cm); jumbo specimens can exceed 36 in
Lifespan
25–50 years
Social needs
group
Native region
East Asia
Origin
Old World
Climate
🍂 Temperate
Water type
💧 Freshwater
Family
Cyprinidae
Genus
Cyprinus
Part of the Koi
Ornamental carp bred in Japan into a rainbow of named color and pattern varieties — large, hardy, long-lived pond fish prized for their top-down beauty and tame personalities.
More koi coming soon.
Habitat & space requirements
From the minimum an animal needs to be kept humanely, up to the ideal setup. Bigger is almost always better — minimums are floors, not targets.
Photo coming soon
Minimum
Garden pond
1000 gal / 3785 L pond
Cyprinus carpio koi reach 24–36 in and live 20–50+ years. 1000 gallons is a strict minimum for a small group (3–5 fish), with strong pond filtration, aeration, and adequate depth (3 ft+) for cold-weather survival.
Photo coming soon
Recommended
Larger garden pond
3000 gal / 11356 L pond
3000-gallon pond with strong filtration, deep cold-weather zone (4 ft+), shade, aquatic plants, and a small group of koi. Koi are POND fish, NOT aquarium fish — never housed indoors long-term.
Ideal
Show-koi pond
5000+ gal / 18927 L+ pond
5000+ gallon professional koi pond with bottom drains, multi-stage filtration, UV sterilisation, depth of 4–6 ft, and 8–10 fish max. Closest to lifelong welfare for these large long-lived fish.
W.carter / Public domain (Wikimedia Commons)
Life & growth stages
How this animal changes through its life — each stage often has its own care, diet and space needs.
Photo coming soon
Egg
Fish eggs are small, translucent spheres, often laid in clutches on plants, substrate, or in a nest — or carried/brooded by a parent in livebearing and mouth-brooding species. A dark eye spot and the curled embryo become visible inside as development progresses.
Photo coming soon
Fry
Newly hatched fry are tiny and semi-transparent, frequently still carrying a yolk sac that fuels them before they feed freely. They lack full fin structure and adult coloration, staying near cover until they can swim and forage on their own.
Photo coming soon
Juvenile
Juveniles look like miniature adults but with developing fins and muted or different markings; many species shift pattern and color as they mature. Growth is rapid at this stage given clean water and steady feeding.
Adult
Adults show the species' full size, finnage, and mature coloration, and are sexually mature. Many fish develop sex-specific differences in size, color, or fin shape, which can intensify during breeding.
Color & pattern variants
Natural variants occur in the wild; selectively bred (man-made) variants were developed in captivity.
Koi are pond fish, not aquarium fish: plan a minimum of roughly 250 gallons (~1,000 L) per adult koi, with established ponds running thousands of gallons and at least 3-4 ft (0.9-1.2 m) of depth to protect from temperature swings and predators. They are cold-tolerant and thrive across roughly 59-77°F (15-25°C), surviving near-freezing dormancy under ice if the surface is kept partly open; pH 7.0-8.5 and moderate to hard water suit them best.
Generous surface area and water movement matter more than current; a gentle to moderate flow with good gas exchange keeps oxygen high in summer heat. Lighting is whatever the sun provides — direct sun intensifies colors but also fuels algae, so partial shade and aeration help.
Substrate
Bare-bottom or smooth river-rounded pebbles are easiest to keep clean; avoid sharp gravel that can injure barbels as koi forage along the bottom. Many keepers run bare liner with a bottom drain so waste flushes to filtration rather than collecting in substrate.
Equipment & setup
Heavily oversized biological and mechanical filtration is essential — pressurized or gravity-fed pond filters, often paired with a bottom drain and settling chamber, plus strong aeration. A UV clarifier controls green water and waterborne pathogens; in cold climates a pond de-icer or air stone keeps a gas-exchange hole open through winter ice.
Diet
Omnivores with a hearty appetite. Feed a quality floating koi pellet seasonally adjusted to temperature: a wheat-germ or low-protein formula in cool water and higher-protein, color-enhancing pellets in warm months. Supplement with vegetables (peas, lettuce, melon), occasional brine shrimp or earthworms, and color-boosting foods containing spirulina or astaxanthin. Stop feeding below about 50°F (10°C) when their metabolism slows.
Behavior & temperament
Peaceful, social, and best kept in groups of at least three to five; lone koi are stressed. They are gentle giants that learn to hand-feed and recognize keepers. Tankmates in ponds include goldfish, orfe, and other coldwater pond species. Koi root in substrate and uproot plants, so water lilies and marginals usually need protective baskets or stones.
Health
Watch for ich (white spot), flukes, anchor worm, fish lice, ulcers, fin rot, and the serious viral threats koi herpesvirus (KHV) and spring viraemia of carp (SVC), which can wipe out a collection. Quarantine all new fish for 2-4 weeks. Most problems trace back to poor water quality, overstocking, or unstable temperatures, so heavy biological filtration and routine testing are the core of disease prevention.
Tips, DIY & hacks
Drip- or float-acclimate new koi and quarantine before adding them to the main pond. Net or cover the pond against herons, raccoons, and cats, and provide deep zones for predator refuge. Test ammonia, nitrite, and KH regularly — koi mature their ponds slowly, so resist the urge to overstock young fish that will grow enormously.
