Snooze wrote:

I've often toyed with that very idea myself. The water in the solar panels
would have to be a closed system. Circulating pond water through the solar
panels would just clog up the collectors with muck and algae, so the bottom
of the pond would need some kind of radiator/heat exchanger.

You didn't say how large your pond is, in a small pond you'd have to be
careful, it's fairly easy to turn a small pond into a hot tub if you run the
solar heater in the summer. I suspect that by the time you add in the cost
of a pump that can raise water a good 20 feet into the air, the solar
panels, the plumbing etc probably cheaper to just add a pond heater.

Its about 1000 Gal.
Yes I expect you are right that the pump and piping required would not
make this a cheap project. That and our local does not get a lot of
consistent sun in the winter when you need the heat.