In the winter much of a pond is dormant but this should not be confused with dead. There is still plenty of life in your pond and to ensure that it stays that way you need to make sure that you keep it a hospitable place to be.
Having an area free of ice is crucial for ponds with fish to allow proper gas exchange. A pond is essentially dormant during cold winter months when biological activity is at a minimum. However, this does not mean there is no biological activity. Fish respiration and metabolic processes still occur, steadily releasing waste products while using up oxygen. Without an opening in the ice, not only do harmful gasses such as carbon dioxide remain trapped under the ice but also the supply of dissolved oxygen steadily decreases. If this condition is to progress and oxygen is not replenished, the fish are unable to survive the toxic environment and fall victim to what is commonly referred to as “Winter Kill.”
An aeration system using an air pump is an effective way to keep water open in the winter. It circulates water to keep a small opening in the ice. This works on the premise that moving water doesn’t freeze as quickly as still water will. It’s important to house the air pump in an insulated box outside the pond, so that it uses dryer, somewhat warmer air than the outside air. An air pump will therefore provide both increased diffused oxygen to your pond and keep a hole in the ice to ensure that gas exchange can take place. Your dormant pond will continue to ‘rest’ through winter but will come out the other side attractive and healthy.