Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
There are special heaters for ponds. The goal it just to keep enough area free of ice for gas exchange. Not to heat the whole pond. You may have to look at pond specialty sites. I do not use one, here. Winters are mild, pond never freezes.
An aquarium, above grade can get a lot colder than a pond in the ground, insulated by soil.
I would wrap 3 sides of the tank in Styrofoam for insulation, and only keep fish that are OK in water that is close to freezing.