I was just wondering if I would have to win the lottery to afford to heat a LARGE outdoor pond for Irridescant Sharks they need their water pretty warm, though my tank has been at about 68-70 all winter and it hasn't seemed to phase them.
What about a solar water heating panal on your roof or around the pond. I was thinking about it the other day...something I did for a school project a long time ago. I pumped water through a grid that was pvc pipe zig zag back and forth all painted black and sitting in the sun. It was on a very small scale but it did heat the water.
For the ponds that I maintain we float a piece of styrofoam with a ring heater in it and an air stone attached to a pond pump running through the ring heater. Keeps the pond heated through the winter and aerated for a low cost. I may be able to get a shot of the setup with my cellphone tomorrow if I am doing an opening, if not friday for sure.
if you build it in a greenhouse you can use the sun to help you . leave a large space for a plant filter of at least 10% of the pond volume and if its in full sun have black pvc running around the outside to help heat the pond ,will not do all the heating but will take a large chunk out of your bill if done right