Pond Heaters

I overwinter my fish in my ponds and therefor need to keep a pond heater going as I am in Zone 6b. Can you give me advice on the type that is most energy-efficient?
Sharon

Doug says that unless your pond is really shallow, you may not require a pond heater. You may get along quite nicely by leaving a pond pump running and pointing at the side of the pond rather than trying to heat up a space. The pump keeps the water flowing and that keeps a space open on the water surface.

But to answer your question - there is no efficient pond heater (they're all electrical resistance units) unless you count moving your pond several hundred miles south :-) to get the sun.