Thanks for all your input. I am working on getting a larger fish tank. The pond is very small with a black plastic liner. It is only about 24" deep in the very center. The goldfish that we stocked it with originally live over the winter easily. We had an ice melter on the top but it just broke this past winter so perhaps a heater may be my next step. It is totally self contained. My husband put the pond in several years ago so I don't see a future problem with local wildlife. I do understand how serious that can be though. The kids really need to understand they can't dump fish in any lake or pond! It could be devastating to the habitat.