Heating a large pond can be very cheap and efficient and even conserve energy. I have learned living in Montana with harsh winters that heated ponds are possible. I kept a 300 gal pond heated to 82 F over winter with temps as low as -40. Electric didn't really bump at all. Used a 500 watt titanium heater. The pond was extremely insulated all around though. I even made a lid that I could open for feedings and water changes that was insulated. I also now have an insulated pond that is 400 gals as my bed stand. I use a 500 watt heater in it as well. I was also able to heat 1000 gals pond to 78 F during the winter by wraping it in insulation as well as a 1000 watt heater and two 500 watt heaters. Would have been more efficient with an insulated lid but did ok with just a tarp.