Fish are pretty easy to care for in the winter. Being cold-blooded, their activity level drops along with their metabolisms. During the cold season, they'll spend most of their time resting safely at the bottom of the pond where water is warmest. The only things they're counting on you to do is ensure their pond stays oxygenated and doesn't completely freeze over. Some ice is fine, it adds a new dimension to the pond that will evolve throughout the season. You just have to keep a small hole open in the ice for unwanted gasses to escape the water.
The problem with the entire pond freezing over is that the gasses released by decomposing organic matter at the bottom of the pond then have no way to escape the pond, which leads to toxic levels of these gasses building up in the pond. Fortunately, preventing that from happening is as simple as maintaining a hole in the ice. This can easily be accomplished with the proper application of heat and aeration.
To learn more about the tools we use to care for a pond through the winter, click here.