Then, I would say look at solar if you have a large amount of sunny days. You could get a small system but it would have to be sized right so it wouldn't have a lot of left over heat.
We might have a week or more of cloudy weather which wouldn't work good without a backup system. I am designing my outdoor pond to be heated by my oil boiler that heats my house. I also thought about installing, an outdoor oil boiler just for the pond. I have also looked at a water source heat pump that would cool my house and heat the pond in the ac months, the pond will still probably drop below 70 without heat during the cool nights of my summer.