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° 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.
There is no need to worry about "frozen fish" if a section of the pond is at least 18 inches deep. Pond fish will seek the deepest part of the pond and over-winter there until the water warms in the spring. If your pond is less than 18 inches deep, the fish may freeze during a harsh winter. Check with your local pond supplier if you live in an area with harsh winters.
Some water gardeners with shallow ponds attempt to keep their koi and goldfish in kiddie pools or aquariums set up in a cool basement or garage. This is not recommended because of the extra stress involved in netting, transporting, and re-acclimating the fish to the new environment.
However, if you dont have a choice, and need to bring your fish inside for the winter, be sure to have an aquarium air pump or small fountain to provide oxygenation. The fish should be fed infrequently, if at all, depending on the water temperature. pH, ammonia and nitrite, which should be monitored weekly and especially if the fish are fed.
Small water changes (20%) each month will keep the water in good shape until spring. Koi are "jumpers"-so be sure to cover the pool with bird netting!
About the Author
Brett Fogle is the owner of MacArthur Water Gardens and several pond-related websites.
He also publishes a free monthly newsletter called PondStuff! with a reader
circulation of over 6,000 pond owners. To sign up for the free newsletter
and receive a complimentary 'New Pond Owners Guide' for joining, visit
MacArthur Water Gardens