I have actually been thinking about that! I think if the waterfall runs into the small pond and then into the main one, it will just drag the heat out instantly into the main pond but I have been mulling over whether there is a way to separate the small pond flow from the main one in cold weather and heat it on it's own, possibly even put a greenhouse or something over it - its certainly a possibility and at this point is the winning idea, although will require a bit more thought!You could build a bigger top part of the pond for the turtles which could also be a filter and waterfall for the bottom pond which would house koi and goldfish then just heat the top part of the pond it will be a lot cheaper and easier.