The common carp is a hardy fish, and Koi retain that durability. They can be kept in anything from small containers to large outdoor ponds. Although Koi can grow to 90 cm (3 ft) if you have one with a good bloodline, the traditional indoor aquarium is far less desirable than a pond.
Koi are cold water fish, so it's advisable to have a meter or more of depth in areas of the world that become warm during the summer. In areas that get harsh winters, it is a good idea to have a pond that is a minimum of 1.5 meters (4 1/2 feet) deep so that it won't freeze solid. It is also a good idea to keep a space open with a pond heater or aerator.
Koi's bright colors put them at a severe disadvantage against predators; a Kohaku looks like a visual dinner bell against the dark green of a pond. Herons, kingfishers, raccoons, cats, foxes, and badgers are all capable of emptying a pond of its fish. A well-designed outdoor pond will have areas too deep for herons to stand in, overhangs high enough above the water that mammals can't reach in, and shade trees overhead to block the view of aerial passersby. It may prove necessary to string nets or wires above the surface.
The pond should include a pump and filtration system to keep the water clear.
Koi are bottom-feeders, so Koi food is not only nutritionally balanced, but designed to float so as to encourage them to come to the surface. When they are eating, you can also check for parasites and ulcers. Koi will recognize the person feeding them and gather around at dinnertime.
They can even be trained to take the food from one's hand. In the winter their digestive system slows nearly to a halt, and they eat very little, perhaps no more than nibbles of algae from the bottom, and their appetite won't come back until the water warms up in the spring.
If kept properly, Koi can live about 30-35 years. Some have been purportedly known to live up to 200 years.