Koi are quite hardy during winters, but they need some preparation to come through it in good health. Fish don't hibernate during the winter, but their metabolism slows down and they enter a dormant period. To help them transition safely, you'll need to change their diet in the weeks before winter weather hits. Get a good pond thermometer because it's water temperature that will tell you when to make changes. Experts disagree on the exact temperatures, but typically you'll want to start slowing down on the frequency of feedings once water temperatures drop below 50 degrees. You can also switch to an easy-to-digest cold water formulation. Once water temps hit 40 degrees, stop feeding altogether. Koi will typically be in full dormancy by then and while you can still see them moving slowly about the bottom of the pond, they're unable to digest or metabolize food they consume.
Overwintering koi need to be kept in a pond that will maintain 2-3 feet of open water below whatever thickness of ice typically forms in your region. Even in a semi-dormant state, they need to be able to breathe and move around a bit. Keep an aerator running near the bottom of the pond all winter to keep oxygen levels up, and use a pond heater to keep part of the surface ice-free so that gas exchange can continue uninhibited. This prevents a buildup of toxic gases under the ice layer. Finally, in addition to tending to the needs of your koi, the pond itself needs a good thorough cleaning to remove any sludge or silt that has collected along the bottom or in the skimmers. With fish in residence, you may choose to add some bacteria that remain active in temperatures down to the freezing point. Keep in mind that many experts consider these supplements a waste of money since the bacteria that prefers cold weather will naturally increase as conditions favor them while others go dormant.
