sounds like a plan. Personally I would make an insulated pond with an insulated lid. You'll have almost no evaporation, and the tank will use much less energy to heat. That's my suggestion for you. I have a cattle trough for just one fish right now that's 3', I would suggest you go way, way bigger. Not sure truely how big a paroon shark can get in a tank since I have not seen any definitive pics of one at 20+ years old. In the wild their numbers are way down, and so is fish size. You have some real potential monsters. I'd probably make a brick or wood pond with thick insulation all around. You'll be fine like that.