The metabolism of koi and goldfish is controlled primarily by water temperature. As the water cools, pond fish require less protein in their diet. When koi and goldfish are fed high-protein food in cool water, the excess protein is excreted as ammonia from the gills. The microscopic organisms that make up the biological filter (and consume ammonia) also slow down in cooler water.

Improper seasonal feeding can lead to a build-up of toxic ammonia, which stresses fish and reduces their winter survivability. When the water temperature drops to approximately 65