If your winters get cold and you want a rtc then you are looking at some money. You would probably need to look at hot tub heaters or a pool heater that is oversized because you will need to keep that water close to 80 degrees. I would also look into some kind of cover to hold the heat in. I had actually thought about doing a heated pond until I saw the cost. Our winters average 20 degrees here. I was going to do a 400 gallon enclosed pond in the garage with all the heaters in it and have it piped to the pond outside so that they constantly recirclulated with each other. The whole idea was too costly for me.