Pond heaters don't heat the entire pond, but they can keep a small circular area from freezing, allowing gas to escape and air to enter the water. This small area is extremely important as it allows for pond gases and air to be exchanged. As waste and organic matter decomposes in your pond, it'll form toxic gases like methane and hydrogen sulfide. This can be harmful for your fish, especially if it's trapped and allowed to accumulate under a layer of ice. Fish also need oxygen just as much as any other animal. Use an agitator or bubbler near the surface to increase contact between the water and the air. However, don't remove all the ice, as this is essential for insulating the water and keeping your fish safe from predators.
It's important to remove the ice via heat, rather than trying to smash or break the ice. Hitting the ice sends vibrations into the water below, and can be dangerous for your fish, harming or even killing them. A pond heater may not be able to keep the entire pond warm, but it can prefent a small area from forming ice and sealing over. Melt the ice via a heater, a de-icer, or by pouring boiling water over the ice. Keep an eye on this hold and make sure it stays open throughout the winter. If you're going to be gone for a few days up to a week, a deicer may be a good idea. It's rarely a good thing to run your deicer 24/7, but for the short term it can be a good way of making sure the ice doesn't reform before you come back.
