Nishikigoi (錦鯉, literally “brocaded carp”), are ornamental varieties of domesticated common carp that are kept for decorative purposes in outdoor koi ponds or water gardens. Koi varieties are distinguished by coloration, patterning, and scalation. The most popular category of koi is the Gosanke, which is made up of the Kohaku, Taisho Sanshoku, and Showa Sanshoku varieties.

The common carp is a hardy fish, and koi retain that durability. Koi are cold-water fish, but benefit from being kept in the 15–25 °C (59–77 °F) range, and do not react well to long, cold, winter temperatures; their immune systems are very weak below 10°C. With proper care, they resist many of the parasites that affect more sensitive tropical fish species. Water changes help reduce the risk of diseases and keep koi not stressed.