Certain kinds of fish are much more prepared to deal with the freezing temperatures of cold winters than others. These fish enter a period of dormancy during the winter where their metabolism significantly slows to prepare them for less opportunities for feeding. To prepare your fish for this dormant period, plan on changing your fishes diet in the weeks before the temperatures start to drop below freezing. The temperature of your pond will tell you exactly when to start making changes. A good rule of thumb is to begin reducing the frequency of your feedings once the water temperature drops below 50 degrees.
Fish will need a few feet of open water beneath any ice that forms on your pond. This will allow them access to oxygen and the ability to move around a bit. Make sure your pond is deep enough to accomodate this! Additionally, consider installing a heater to keep part of the surface of your pond ice-free. This will allow gases to pass freely between the water and the surface, preventing gas from bulding up underneath the ice. An aerator at the bottom of your pond should be able to run throughout the winter. This provides vital oxygen to your fish so that they're able to thrive even in the winter.
