Most fish have no problem surviving the cold water temperatures that come with winter weather. However, fish in ponds face a different set of problems. Temperatures in shallow ponds can become too low for the fish to survive. Vegetation dies, lowering oxygen levels in the water and leaving no place for the fish to hide from predators. When a pond completely freezes over, organic waste that produces toxic gas becomes trapped in the pond, smothering fish. With so many dangers, many people move pond fish to indoor tanks for the winter, but other methods to care for the fish without relocating them are available.
Clear snow from the top of the ice. Snow blocks light needed by microscopic pond plants to produce oxygen. Fish need the oxygen to survive, and allowing light to reach pond plants prevents suffocation.
De-ice an area of the pond to keep oxygen levels high enough for fish to survive. Floating pond de-icers containing a thermostat are available at many garden and pool stores. The built-in thermostat turns on a heating element when water temperatures reach a certain level, keeping ice from freezing over. The open area allows toxic gases to escape. Place de-icers in the shallowest part of the pond, and shelter them from the wind.
Aerate pond water with an air pump or use a submersible water pump to keep an area of the water free of ice. Placement of the pumps is critical to maintain necessary oxygen levels, temperature and water quality. Aquatics Unlimited recommends placing pumps in the centre of the deepest part of the pond for best results. Aquatic supply store Foster and Smith suggests keeping the aerator pump outside of the pond in an insulated chamber and placing only the airline and air-stones in the middle level of the pond.
Thaw areas of the frozen pond using bags or containers of boiling water. Place the containers and bags of hot water on a shallow section of the pond to melt the ice.
Feed fish wheat germ-based fish food about twice a week. Fish don't eat as much in the winter and wheat germ is easier for them to digest. When water temperatures drop below 10 degrees C, stop feeding the fish.
Place a leaf net over your pond to keep falling leaves from polluting the water. The net can also protect the fish from predators.
Consider increasing the depth of part of your pond to at least 36 inches to protect your fish from long periods of cold. Purchase a generator to keep your de-icer and aerator operational during power outages.
Do not break or pound on the ice, as resulting shock waves can kill or cause damage to your fish.