Koi, sometimes Goi or Nishikigoi means carp. They are probably the most popular pond fish. This type of pond fish allows for more human-pet interaction than most. You can feeding them with your hand, pet them, and rub their bellies. They can grow to 30 inches long in a good size pond. They are beautiful but yet very strong and sturdy. They can survive through severe cold and hot temperatures. They can live under ice provided that the pond is not completely frozen solid.
Koi should be loved for their beauty. Grouping them is only for: the sake of contests. Usually there are two main categories; "Basic Varieties" and Unique Koi. Two basic body types: the medium body depth like the Magoi and the much deeper body like that of the old Koitsu-goi. Mainly the grouping is according to the color, color patterns and scale types. There are roughly 13 basic groups (Kohaku, Taisho Sanshoku, Showa Sanshoku, Bekko, Utsurimono, Asagi Shusui, Koromo, Kawarimono, Ogon, Hikari Moyomono, Hikari Utsurimono, Kinginrin and Tancho) and more than a hundred of Unique Koi.
Koi is magnificent but also can be very expensive; million dollar Koi is not uncommon especially in the far east countries. On the other hand, you can keep our reasonably priced, best valued Koi and enjoy them just as much as the very expensive ones. But I should caution our hobbyist that it can still be very costly if your fish are routine victims of predators in the backyard such as birds, cats or other unexpected visitors.