MC wrote:

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

You're kidding me? I could have sworn I had a hole in the ice surface all
winter. A bubbler will work in any weather I've had to encounter in zone 6
- for you, you might have to resort to something else for three or four
days.

A 300 watt heater will raise the water temp 10 degrees for a 300
gallon pond as a rule of thumb.

That might just barely work in an aquarium in a house, where you're not
dealing with a solid-gas phase change. Remember, it takes 40 times the
energy to change ice to water as it does to raise the temperature of water
1 degree.

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.

Even if I were to accept your numbers, 600W x 24hours x 50days (my guess at
the number of sub-freezing days you're going to be faced with in Zone5) =
720KWh to run those heaters over the winter. Considering that I currently
have an _annual_ electricity consumption of about 500KWh, and your average
household uses 100-200KWh, monthly, that sounds completely out of whack.
--
derek