I feel your pain last year was my winter with my pond and I was stuck in the same situation. I have a 900 gal pond 24" deep. It depends on what zone your in as to what kind or size heater you can choose to use. I live in southeast pa and we had a brutal winter last year. My saving grase was that I left my pump run all winter. I moved my waterfall supply hose to poor into my pond from 6" ubove the surface of the water and that kept the pond surface from completly freezing allowing dangerous gases to escape. I also tryed a 250w surface deicer but that didnt seen to work so well, I dont think it was large enough enough. Trough heater do work but you need to protect to fish from the heating element. I dont know if I did everything right but all my fish made it through the winter. As for the health of a winter enviornment, I just think of fish in a natural pond. The thing they have going is the depth of the water and you are at 36". I just let the water get cold because thats what happens in a natural pond. I just keep the surface from completly freezing because of the pond liner, so a surface deicer is the best way to do so.

Here is a link that might be useful: Winterizing Ponds

Thanks jr for sharing your experience and the link...both very helpful!

Hi Melissa,
A while back we had a discussion about wintering fish over. I've posted the URL below just to save my old fingers a bit of retyping. As to using a heater...for the most part heaters don't really do that much for changing the temperature environment of the pond. Yes they will keep a hole open but even the largest heater isn't going to raise the temperature too much. This, of course, depends on the zone you live in or rather how cold it gets. The first person in the posting/thread listed below, Jenny, provided a link to an excellent article on the subject...'tis worth a read (I've checked and the URL is still valid so far). Hope all this helps.
---David

Here is a link that might be useful: Posting on over-wintering Koi under thick ice