Many people misunderstand the role of a pond heater. It is the rare pond that can actually be heated during winter ice and snow. It would be far too expensive to heat anything larger than a small self-contained porch pond. Even that might be an expensive task.
As long as there is still liquid water at the bottom of the pond in which the fish can swim, fish don’t need the heat. Goldfish and Koi are cold-blooded and survive quite well near freezing temperature. Their metabolism goes down in the cold months anyway. They need very little oxygen, very little food and very little room to move about.
However, when backyard ponds ice over, harmful gasses from decaying plants, algae and fish waste cannot dissipate. The environment begins to become toxic as ammonia and nitrites rise. At that point, a pond heater is needed to clear a hole in the ice.
A Thermo Pond Heater floats on the surface of the pond, heating a small space around and under it. This creates an opening and allows the gas/air exchange that keeps these toxins from building up. It costs very little because it is heating only the smallest area around itself. Fish stay healthy all winter long because the toxins are allowed to escape.
If you have a particularly large pond, you might need to purchase two or more heaters to spread over the surface area. The gasses will travel up, but currents are usually minimal, and they may not travel horizontally toward a distant hole.
