MC wrote:

-25C is about -13F. You must get great sun and be protected from the
wind. I've seen 4 foot high waterfalls freeze in temps that cold. How
does a small air bubbler keep the water from freezing?

In S. Ontario, with temperatures down to -25C, I could keep a hole open
with a 15W aquarium air pump and one of the long (6") air stones,
suspended 6-12" below the waterline.

I don't have a clue :-) I do know that they use bubblers in the Great Lakes
to keep marina berths ice free, and for some of the ferries on Lake
Ontario.

I did have great sun, but I wasn't protected from the wind. In stormy
weather you'd sometimes get enough slush to block the hole faster than the
bubbler could clear it, so it requires a little manual assistance, but it's
storms (generally at close to freezing temps) not extreme cold that are the
bigger problem with the bubbler.
--
derek