A bubbler won't do anything in really cold weather. The water freezes.
Even running a waterfall, it will freeze in really cold weather.

A 300 watt heater will raise the water temp 10 degrees for a 300
gallon pond as a rule of thumb. I am planning on 2 300 watt heaters.
That should raise the temp about 15-20 degrees with the solar cover.
I would expect at 30 inches below ground with the ground acting as an
insulater, It will keep the temp above 40.

Derek Broughton wrote in message ...
MC wrote:

2) I've read bio filters are useless below 50 degree. So I won't run
it. Is it better to remove it from the pond, or just leave it? I
anticipate the heater will keep my pond around 40. I don't intend on
"heating" it, just keeping it from freezing solid.

40? _That_ is some amount of heat. If an aquarium heater works at all,
your surface temperature is going to be within a degree or two of the
freezing point. Forget the heater, use a bubbler.