Different kinds of fish will be able to handle the cold better than others. Certain kinds of fish are even able to handle temperatures up to or below freezing! Koi, goldfish, bluegills, archerfish, redears, and rosy red minnows are just a few examples of the kinds of fish that are more likely to be able to withstand colder temperatures. In general, these fish will require at least 2 to 3 feet of open water underneath any ice that may form on the surface. This provides them oxygen and room to move around and live.
If you're worried things may get too cold for your fish in the winter, consider heating the water in your pond. This will prevent ice from forming and keep things warm. Be aware though that this may prevent your fish from entering dormancy in the winter, and they may require regular feedings.
