You could use one of those bucket heaters, would expensive though.

Jerry Cloe wrote:
I plan on setting up a somewhat small pond (probably around 7500 gal's, but
less than 4 feet deep). I realize that the surface freezing over isn't a
problem, but I'm worried about it freezing completely through.

I'll have a pump to generate a small waterfall (and will have filters setup
at that point).

My question is should I install some type of small water heater or something
and heat the water (probably set at 55 to 60) so as the temps start falling
below the water will be maintained.

Does anyone have any ideas or input on this? Reading through past posts I
don't really see that much about heating.