Very interesting fishtank idea. Seems that most people that have a hot tub that works usually uses it for themselves, Tell you what, I will trade you your working hot tub for a 3' above ground swimming pool. Oh wait, I see that you posted this in August, perhaps now that winter is amongst us, you might have changed your mind. Seriously though, as already stated, even if not heating a full capacity, it might still be pricy. Avoid the copper, and make sure no chlorine or other chemical leftovers in the system, and it should do fine. I assume that hot tubs would be well insulated?