Koi are carp (Cyphnus carpio) and were first mentioned in Japan some 1600 years ago, when coloured varieties were kept for decoration in ponds. Over the years these fishes have been more fully developed by careful breeding and selection, to give us today a wide variety of forms and colours. They are particularly popular for garden pools because of their large size, brilliant colours, long life, character and stamina.
They can be seen easily because of their size and vivid colours. They also grow quickly, and may become sufficiently tame to eat out of your hand. Koi can be left in the pond through the winter, provided the water remains just above freezing point. As they are bottom-feeders, they will stir up any soil that is exposed, clouding the water; it is therefore important to cover all the soil with gravel or stone chips.
Koi can grow to some 90cm (36in) long and need plenty of water to swim in, with a minimum depth of 1.5m (5ft) to give the fishes a frost-free area in winter. If you live where temperatures are likely to be consistently below zero, a cover for the pond and a heater are advisable. When calculating the number of fishes, allow at least 2.5m2 (27ft2) for each Koi, to allow for growth and for sufficient food to be produced for it. Some water movement is important to keep the oxygen level high, particularly in the early morning and in hot stormy weather.
Although Koi can be sub-divided into the two groups of Japanese and German Koi (the former slimmer than the latter), the main classification is by colour and pattern. The colour is derived from the basic pigments in the fish and can be black, white, yellow, blue or red.
Shiro Muji is a white Koi, pure and unmarked.
Aka Muji is red.
Karasu Goi is a black Koi, but some have white markings.
Asagi has a back covered with blue scales with a lighter blue between, giving it a netted look.
Kohaku (Red-white) is basically a pure white fish with deep red markings, the edges of which should be sharp and clean; the markings preferably start with a red spot on the head and finish with a red spot on the trunk before the caudal fin. There are various sub-groups, defined by the nature and balance of the markings.
Taisho Sanke (Japanese Tricolour) is a pure white Koi with red and black patterns; there should be a large red spot on the head, and the patterns must be clear and distinct.
Showa Sanke (Modern Japanese Tricolour) is a black Koi with red and white markings. The main difference between this and the previous group is that the main colour has to wrap right round the trunk of the fish.
Again there are sub-groups within this section, depending on the type, size and colouring of the patterns.
Ohgon (Gold) is a pale overall gold. Yamabuki Ohgon (Yellow Rose Ohgon) is a cross between a yellow Koi and an Ohgon; it is a spectacular fish with metallic scales.
Hariwake Ohgon is a patterned fish with clearly divided patterns of gold and silver, and of a metallic sheen.
Shiro Utsuri is a black Koi with white markings that make a bold and distinct pattern.
Ki Utsuri and Hi Utsuri: the former looks like the Shiro Utsuri but with yellow markings instead of white ones, and the Hi Utsuri has red markings on the black base.
Kinsui and Ginsui are metallic-looking fishes with either gold (Kinsui) or silver (Ginsui) overall colour. These Koi are spectacular but the brilliance fades after a year or two.