New pond owners often inquire about the health and safety of their fish during the winter months. As long as your pond does not freeze to the bottom and an air hole is provided on the pond’s surface, your fish will survive the winter.
If your pond is at least two-feet deep, the proximity of the earth to the pond’s surface will keep the pond from freezing any deeper than eight inches. That leaves 16” for the fish to lounge and hibernate over the winter.
Even though your fish are in hibernation, they still need oxygen to survive the frosty months. If you turn your waterfall off for the winter, you’ll need to supply oxygen with a pond aerator or a small recirculating pump.
Place the aerator or small pump on a shelf in your pond. Avoid placing it at the bottom as this disrupts the pond’s thermocline during the colder months. An aerator or small pump will add oxygen while helping to maintain a hole in the ice.
If you live in an area with several months of freezing weather, you may need the additional help of a pond de-icer to maintain a hole in your pond’s ice. A hole in the ice allows for the exchange of gases so that the water doesn’t become toxic to your fish.
Your pond fish will become dormant during the winter once water temperatures drop below 50 degrees F. This is also the point at which you should stop feeding them. The enzymes needed for the digestion of most koi food is lacking once the weather turns frosty. You don’t want your fish to eat and then languor in the cold water as their metabolism slogs the food through. In very cold water, fish simply don’t eat.