go get a powerhead, a submersible 300 watt heater, and a piece of pvc the heater can slide into with enough room around the outside of it for water to flow freely around it. attach the powerhead to one end of the pvc, then slide the heater in from the other side, this way the water is constantly moving over the heater keeping the heat spread through the pond. if it isnt enough then quickly make another one and drop it into the pond. between the movement of the water, and 1 or 2 300 watt heaters it shouldnt freeze. Insulating the top will only help things.