Planted Tank Obsessed
Join Date: Sep 2010
Location: havre de grace, MD
Not sure how brave you are, or how good you might be with electricity, but... the home depot 1-2-3 manual would more than likely have enough information in it to help you run some direct-bury romex (electrical wire that can be buried straight in the ground) from a power source in your home, to someplace near the pond. Then you could put in a post, with an exterior outlet, and plug in a heater. It might sound like a bit much, and rather scary (as a lot of people are afraid of electrical work) but it really is quite simple. And as long as you keep circuit breakers off, follow the steps in the book, and ask your local home depot for their "electrical guy" for a little advice, you can wire it in safely.
I re-wired an entire house with very minimal previous electrical experience. Followed the book, asked for advice, and when all was said and done, I had it checked out by an electrician, and the city inspector to make sure everything was good to go. They both said that it was some of the best work they'd seen.
I know this isn't the answer you were looking for, but don't discredit it right away. It might make for a good spring or summer weekend project.