I've done similar things for hydronic heating in my greenhouses, and I've been amazed at how much heating the water will heat the greenhouse itself. I'm sure you know already, but make sure that none of the fish water comes in contact with any of that copper tubing. Copper is very expensive; I've used pex and even polyethylene where the temperature wasn't going to be too high. Dropping a coil of poly tubing into the water itself provides quite a bit of heat exchange, though, obviously, it would require some testing.
