Health, Maintenance & Longevity
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. Koi ponds usually have a metre or more of depth in areas of the world that become warm during the summer, whereas in areas that have harsher winters, ponds generally have a minimum of 1.5 metres (4½ feet)
Koi's bright colours put them at a severe disadvantage against predators; a white-skinned Kohaku is a visual dinner bell against the dark green of a pond. Herons, kingfishers, otters, raccoons, cats, foxes, badgers and hedgehogs are all capable of emptying a pond of its fish. A well-designed outdoor pond will have areas too deep for herons to stand, overhangs high enough above the water that mammals cannot reach in, and shade trees overhead to block the view of aerial passers-by. It may prove necessary to string nets or wires above the surface. A pond usually includes a pump and filtration system to keep the water clear.
Koi are an omnivorous fish, and will eat a wide variety of foods, including peas, lettuce, and watermelon. Koi food is designed not only to be nutritionally balanced, but also to float so as to encourage them to come to the surface. When they are eating, it is possible to check Koi for parasites and ulcers. Koi will recognize the persons feeding them and gather around them at feeding times. They can be trained to take food from one's hand. In the winter, their digestive systems slow nearly to a halt, and they eat very little, perhaps no more than nibbles of algae from the bottom. Feeding is not recommended when the water temperature drops below 10°C (50°F). Care should be taken by hobbyists that proper oxygenation and off-gassing occurs over the winter months in small water ponds, so they do not perish. Their appetites will not come back until the water becomes warm in the spring.
One famous scarlet Koi, named "Hanako", was owned by several individuals, the last of whom was Dr. Komei Koshihara. In July 1974, a study of the growth rings of one of the Koi's scales reported that Hanako was 228 years old. The greatest authoritatively accepted age for the species is little more than 50 years.