As most of you know I am building a 8'x3'x2' Plywood/pond liner pond for my large cats, I have run into a snag however, how should I heat the tank. I assume heaters can be used if they are out of the way from the crushing power of my large babies, just how many do I need, what wattage, and which would be best for my application? The pond is a little under 400 U.S. gallons. Also, my house stays a constant 70-72 degrees F at room temperature, if that means anything. Many thanks, it's much appreciated.