If you have cold-hardy fish like Koi or common goldfish, they'll do just fine in a properly prepared pond. The first step in setting up a well-prepared pond for winterizing is in the design. Good job on considering this early! In regions where winter gets quite cold and surface waters are expected to freeze solid, there are a few basic points to incorporate into your design. First, make your pond deep enough that there's adequate swimming room even at the maximum ice depth. Two feet minimum is a good benchmark.
Next, install a good aeration system near the bottom of your pond throughout the year to keep oxygen levels up. A pond heater can be installed to keep part of the surface ice-free so that oxygen and other gases can pass freely between the water and the open air. This helps prevent gasses from building up underneath the ice and harming your fish.
