koi

Koi are ornamental varieties of the common carp (Cyprinus carpio) kept for decorative purposes in outdoor ponds. Although associated with Japan, koi were developed from carp varieties in East Asia and gained widespread popularity in the 20th century. They are not a single species but a group of domesticated color morphs selected for pattern, scale quality, and body shape.

Biology and varieties: Koi are freshwater fish that can grow to substantial sizes in well-maintained ponds, often reaching 60–90 cm or more and living for several decades with proper care. They are hardy fish, tolerant of a range of temperatures, though extreme heat or cold can stress them. Classic varieties include Kohaku (red and white), Taisho Sanke (three colors with black), Showa Sanshoku (three colors with black on the body), and metallic or patterned types such as Ogon and Asagi. Breeders produce numerous color and pattern combinations, resulting in a wide array of koi.

Care and husbandry: Keeping koi requires a large, well-filtered pond with stable water quality, adequate oxygenation, and protection from predators. They are social and commonly kept in groups. Diet typically consists of commercially formulated koi pellets, with occasional vegetables or fruit; overfeeding should be avoided. Regular monitoring of water parameters and periodic partial water changes help prevent disease.

Breeding and health: Koi reproduce in ponds under suitable warm conditions, laying many eggs that hatch in several days. They are susceptible to diseases such as koi herpes virus (KHV), ich, and parasites, so quarantine of new fish and good biosecurity practices are important.

Cultural significance: In Japanese and broader East Asian culture, koi symbolize perseverance, strength, and good fortune. They are featured in garden ponds, art, and koi shows. Releasing koi into non-native ecosystems can be ecologically harmful and is discouraged in many regions.