ThermaKoi Electric Heater
Simple to use, 110Volt normal plug in, you simply pump water in one end and it flows out the other back to the pond. So you can either put this in line on your filtration system or you can dedicate it a pump of its own. This unit runs on electricity and pulls about 12 Amps while operating. It comes equipped with a flow switch to turn it off if water stops moving, which will spare the element. It's all stainless construction and it's reasonably priced.