I can't recommend a heater for you since I have never had an outside tank. I live in Michigan, and I know people with ponds. Their fish vacation inside in the winter. I know if it were me though, I would have a sump with the heater in there either inside, or at least in a garage that would help offset the outside temps so that the heater wouldn't have to work as hard. Not sure if that's possible for you, but it's food for thought anyway.