As a product of artificial selection (man made), Koi fish don't occur naturally in the wild, as the other carps. In this context, the usually man-made koi pond has become the "natural" habitat of koi fish.
If koi becomes a feral fish, which "escaped" to the wild, it is likely to crossbreed with carps. In this case, its colorful features are not fully transmitted to the next generations.
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History
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Carps were an important source of protein for the Japanese rice farmers during the 19th century. The shallow rice paddies were also used to raise the fish, which was introduced in Japan during the Chinese invasion, in A.D. 200, according to Absolute Koi.
When the farmers noticed that some carp had red and white patterns, they decided to keep them for decoration, and later crossbreeding. Modern day color patterns were determined by 1880, when Koi breeding was already a pastime.
Nowadays, instead of rice paddies, decorative ponds around the world are the natural habitat of koi.
Geography
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According to the National Institute of Water and Atmospheric Research, koi fish can be found on every continent except Antarctica, as they are very adaptable.
In some countries, the spread of feral koi fish has caused problems. Koi fish can increase the turbidity of water when they feed, stirring the substrate of the rivers to find plants and small animals. This can make water unsuitable to human and livestock consumption. According to the National Institute of Water and Atmospheric Research, while such damages have not yet occurred in New Zealand, the spread of feral koi fish should be minimized.
Growth
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In their natural habitats and at optimum conditions, carps can reach an average length of 2 feet and a weight of up to 2 pounds, as stated by Fisheries and Aquaculture Department of Food and Agriculture Organization. In artificial ponds, the koi fish's development is extremely dependent on the physical and nutritional conditions of the water.
Considerations
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Fish ponds are today's substitutes for the old Japanese rice paddies. However, as man-made environments, such ponds should resemble the originally wild habitat as much as possible.
According to Absolute Koi, the basic requirements to keep healthy koi fish include a outdoor pond (made of concrete, fiberglass or butyl liners) and a filtering and cleaning water system.
Care
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To the Olympic Koi, Goldfish &Water Garden Club, koi ponds are sometimes too dissimilar to the wild habitat, as they are man-made, have a larger population relative to water volume and unnatural food sources.
In an article published on the Koi Club of San Diego's website, crystal clear water is not always the best option for the fish. Green turbid water can better reproduce the natural environment and be beneficial to the fish.
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