I gave up that battle after last winter, going out and trying to reopen my
frozen bubbler hole, and still losing my fish. This year I put up a hoop house
(just 6 mil plastic over pvc) and it has worked like a charm. I've stuck my
head under the plastic a few times and it is all clear of ice. I convinced
that this is way to go. I'm in Iowa, zone 5, and my pond is actually shadowed
by the neighbors house, so I'm not getting the full greenhouse benefit, yet it
is working great. This is a lot less cost and worry than a heater.

Dave