Water temperature affects fish more than any other single factor. Fish are ectothermic - their body temperature fluctuates in accordance with the temperature of their environment, usually remaining 10C higher.
As the temperature drops, the ability of the koi to digest and assimilate food decreases. In the winter months, therefore, at temperatures below 10C(50F), it is a good idea to feed cereal diets that the koi can digest quickly and easily and that do not stay in the gut too long. High-protein diets linger in the gut and can cause severe problems; the bacteria found in the fish's gut, which play a role in breaking down some less readily digestible substances, such as cellulose, may become pathogenic if food is retained in the gut too long.
As the water temperature rises, however, the koi need protein for growth, repair of damaged tissue and injuries, and for reproduction. In the summer, koi will benefit from a high-protein diet containing 35 to 40 percent of fishmeal-based protein.