Koi

Cyprinus carpio

Koi are ornamental domestic forms of the common carp, Cyprinus carpio, developed especially in Japan as nishikigoi, or brocaded carp. They are the same species as common carp but are selected for color, pattern, body shape, skin quality, and growth. Classic varieties include Kohaku, Sanke, Showa, Utsuri, Ogon, and many metallic or scaleless forms, with red, white, black, yellow, blue-gray, and orange combinations. Individual koi can become large, long-lived pond fish, and serious breeders evaluate them very differently from ordinary goldfish or feeder carp.

Keeping koi is mainly pond management. They need excellent filtration, aeration, stable water chemistry, enough depth for temperature swings, and protection from predators. Feeding is adjusted to water temperature, with higher-protein growth diets used only when the fish can digest them well. New koi are commonly quarantined before joining a pond because parasites and viral diseases can spread quickly. Breeding programs produce many fry and then select heavily for pattern and structure. Because common carp can damage natural waterways, unwanted koi should be rehomed responsibly and never released.