Heating Garden Ponds - Tech Talk 123
Koi, goldfish, catfish, minnows, bluegills and many other fish are capable of living in ice-covered ponds in their natural conditions. But when you put a lot of fish in a small pond, a fishkill will result if the ice cover persists too long. To prevent a fishkill, all you need to do is keep a very small percentage of the surface ice-free for gas transfer. Use one or more small heaters with thermostats built for the purpose.
Water temperatures below 65°F (18°C) can stress fish, reducing their resistance to disease. Rapid and extreme fluctuations can also cause disease. If your fish are extremely important to you, and you want to keep the temperature above 65°F, you will need to both insulate and heat your pond in most locations in the US. Unless you don't mind spending a king's ransom for heating, you will need to insulate and turn off all fountains, waterfalls and other water-chilling devices.
Insulation can take the form of floating styrofoam sheets, swimming pool cover, greenhouse structure, plastic sheeting, etc. Keep in mind that when water evaporates it has a significant cooling effect. This effect can account for as much as 80% of the total heat loss from open ponds. Be prepared to open the insulation on warm days; otherwise, excessive heating of the water can take place, shocking the fish.
It may be more cost-effective to transfer fish to indoor tanks with recirculating filter systems for the winter. This also allows for winter pond draining and maintenance.
The heat loss in BTU/hr per square foot of pond surface is approximately 15 times the temperature difference between the desired water temperature and the coldest average air temperature. Example: If you wanted to maintain water temperature at 65°F, and you knew that the air temperature was going to drop to 35°F (24-hour average), you would have a 30° temperature difference x the surface area (which, in this case, we will say is 100 sq.ft. x 15) = 45,000 BTU/hr. If you insulate 80%, your heat loss will be less 80%.
What are the heating equipment options? Immersion heaters that are outdoor-rated or submersible may be used, as well as the electric flow-through type. Gas heaters of the type used on hot tubs and swimming pools typically are more cost-effective for the large heating loads. Beware of copper or cuper nickel piping, which makes them incompatible with most fish cultures. They can be used, however, with stainless steel or titanium heat exchangers. The water should be filtered prior to entering the heater to avoid biofouling.