I am not sure what the cost is associated with heating 300 gallons, but with decent insulation, it shouldn't be too bad!

It sounds like you are saying you can't use any of those fish for your climate. I have known people up north to simply pack up their setup during those colder months and start back up when things warm up. There is a youtube poster that has a heated greenhouse (fully automated with everything plugged into a computer, very neat). Otherwise, I think you will just have to work with mother nature on this one.