it's done in alot of places . there are tanks that are done this way. I have been wanting to do an outside saltwater pond for a long time but not exactly sure about whether I want to mess with it or not. I would do geothermal for sure. bury some 55 gallon plastic drums in the ground and your set. all day direct sunlight might be tough and I could always build myself a heat pump for the winter. I was so close I almost ordered a liner about a year ago. price of electricity is the down fall. big closed loop and a skimmer and you would be done. if anyone has thought about this I have done alot of research on it.