A heater can prevent ice from forming in some areas, this allows gases to escape and for oxygen to permeate throughout the water. This is an incredibly important part of maintaining your fishes' health and your pond's water quality in the winter time. As plants die back during the colder months, their ability to filter out toxins is all but eliminated.
As organic material decomposes, it releases toxic gases in the form of sulfur dioxide, ammonia, and CO2. These gases can suffocate fish and plant life and severely reduce your water quality. Typically, these gases escape from the water's surface into the open air. However, if your pond is completely encased in ice, theres nowhere for these gases to go.
