Koi

Quick facts

Adult size

60 cm, 1500 g typical harvest weight

Days to harvest

1095 to 3650 days from fingerling

Lifespan (max)

up to 50 years

Diet

omnivore

Temperature class

warm-water

Difficulty

intermediate

Water parameters

Temperature range

4–30°C (optimum 22°C)

pH

6.5 to 8.5

Hardness

5 to 25 dGH

Minimum tank

1000 L per individual at harvest size

Feed and growth

Feed protein

30% target

Daily feed (warm water)

2.00% of body weight per day

Daily feed (cool water)

0.50% of body weight per day

Max stocking density

30 g per litre of system water

A 1500g adult eats about 30.0 g of feed per day at optimum temperature. For a roster of 10 fish at adult size, that's around 300 g of feed daily.

Legality

Aquaculture and possession rules vary by jurisdiction and change over time. This table reflects regulations as of the verified date on each row. Verify with your local fisheries or wildlife authority before stocking.

Jurisdictions not listed here default to "check local regulations". A non-listing is not a green light; rules in your specific county or municipality may apply.

Habitat and origin

Koi (Cyprinus rubrofuscus, sometimes classified as C. carpio) are ornamental color variants of the common carp, developed through centuries of selective breeding in Japan beginning in the Niigata region in the early 1800s. Koi are genetically the same species as common carp but are bred for color, pattern, and body shape rather than food production. The dozens of recognized koi varieties (Kohaku, Sanke, Showa, Ogon, etc.) command prices from a few dollars for pet-grade fish to millions of dollars for champion specimens. Koi are typically considered ornamental, but they're included in the food-grade section because they're biologically identical to common carp and functionally interchangeable in aquaponics systems. Adults reach 60–90 cm and 5–15 kg. Koi are extremely popular worldwide, with major breeding industries in Japan, Israel, and China.

Climate and outdoor ponds

Climate classification

temperate (handles seasonal swings)

Outdoor pond zones (USDA)

3 to 11 (winter low around -40°C or warmer)

Heating in a temperate climate

Not required (handles seasonal cool periods)

Cooling in a temperate climate

Not required

Zone bounds reflect year-round outdoor pond viability with no active heating. Anywhere outside the bounded zone, the species can still be kept in an indoor heated tank or a seasonally-managed system. Verify your specific microclimate, as a sheltered yard zone can run a half-zone warmer than the regional rating.

Care notes

Koi function identically to common carp in aquaponics: they produce substantial waste for plant nutrition, tolerate a wide range of water conditions (pH 6.0-8.5, temperature 4–30°C, dissolved oxygen above 3 mg/L), and eat commercial pellet feed (25-35% protein) readily. FCR is 1.5-2.5 on koi pellet, similar to common carp. Growth to aquaponics-useful size (1–3 kg) takes 12-24 months. Stocking density: 15-30 g/L. The twist with koi in aquaponics is that the fish themselves may be more valuable alive than as food. A healthy, well-colored koi is worth $10-200 alive (far more than any food fish of the same weight), which means aquaponics operators can potentially sell surplus koi to the ornamental market for more revenue than harvesting and eating them. This dual-use approach (koi as nutrient generators for plants AND as a cash crop sold alive to the pond/aquarium market) is increasingly popular. The main concern is that koi are common carp and are regulated as invasive species in some jurisdictions (particularly Australia, where they're a declared pest). In the US, koi are legal in most states but may require containment protocols to prevent escape. Fingerlings are widely available from koi breeders and pet stores.

Further reading

• Fish for aquaponics: tilapia vs everything else