Over-wintering pond fish: "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� F, start feeding with Spring & Autumn Pond Food. This type of fish food is better suited for the dietary requirements of pond fish in cool water and won’t pollute the water with excess ammonia. Some water gardeners continue to feed their fish until they no longer come to the surface. I stop feeding my pond fish when the water falls below 42� F. "