Koi are not hard to keep as long as they are kept in a pond or other similar body of water. They easily adapt to cooling temperatures, going into a period of dormancy during the winter when they require no food. Part of keeping koi is properly caring for their habitat.
Keep koi in a man-made pond that has a filtration system to manage algae growth and remove waste products. Mechanical and biological filtration systems work well, the latter of which is aerobic bacteria that eats away at the waste in the pond. Algaecides or ultraviolet filtration can help to control algae growth. Some natural ways to control algae include placing the pond in the shade, which keeps algae from using the sunlight to grow; adding salt to the pond; and adding koi clay to the water, a natural mineral-rich substance that helps koi to thrive while inhibiting algae growth.
Koi need to be fed regularly depending on the water temperatures. For instance, koi require no food if the water temperature reaches above 85 degrees Fahrenheit. If it drops under 40 degrees Fahrenheit, they go into dormancy and also should not be fed. For temperatures between 81 and 85 degrees, the fish need a protein-based food such as unpeeled prawns, krill or earthworms every day. For temperatures between 66 and 80 degrees, feed the fish a protein-based food from one to three times a day. At temperatures between 61 and 65 degrees, the fish needs protein-based food and wheatgerm four or five times a week. The koi only needs to be fed a wheatgerm-based food one to four times a week when the temperature drops between 45 and 60 degrees.