Depends on the type of bass. Don’t think you’d need one if the pond is deep enough and if your talking about regular bass. If it’s a peacock...it’s gonna cost you a ton in heating bills. You just wanna make sure the top doesn’t freeze over for proper gas exchange. You can get a floating pond heater or a large titanium heater. Put it on the shallow side and float something over it like styrofoam or large plastic lilies to keep the heat from escaping. It’ll make a nice warm spot for the fish. Ask the local pond store for advice