How to Feed Koi. Like any fish you take care of, a koi fish has to be fed. Environmental factors, such as cold or warm water, determine how much your fish will eat. The types of food and how much it eats need to also be based on its size.
Take the water temperature that your koi fish is living in. This is important because cold water slows the digestive system of the koi and it can't digest certain foods.
Feed your koi in cold water. For water that is less than 70 degrees Fahrenheit, feed it once per day or every other day. When the temperature lowers to 64 degrees Fahrenheit or lower, feed it every other day. Don't feed it when the temperature is below 53 degrees Fahrenheit. Another option is a cereal-based diet because the cereal will break down quickly.
Feed your koi in warm water. Koi below 8 inches in length should eat around 5 percent of their body weight, while koi over eight inches in length should only eat around two percent of their body weight. Another way to determine how much food to give to your koi is to limit the amount of food that you offer to it, to how much it can eat in about five minutes.
Provide the nutrition your koi fish needs. A healthy koi fish often has brighter colors than a malnourished one. While you shouldn't feed them a diet of live foods alone, your koi will enjoy the treat. Earthworms are a year-round food and a favorite of koi. They also like cockles and prawns. They will eat most food you offer them, but a lot of things aren't nutritional and can harm them. Use koi food in pellet form for the main part of their diet. These pellets will give most of the vitamins, minerals and proteins that they need.