There is a type of swimming pool out there called a Natural Swimming pool were they take a existing swimming pool and convert it into a pond but they had special filterers to it to keep the water moving and the ecosystem in the pool in balance. the nice thing about natural swimming pools is that some of them can have fish in them and you can swim in them along side the fish if they are properly laid out and built. I'm personally thinking about building one with some large viewing windows on the side of it so my fish can use it the 92% of the time that I don't go swimming.
Here is a link on wikipidia http://en.wikipedia.org/wiki/Natural_pool
Here is a link on wikipidia http://en.wikipedia.org/wiki/Natural_pool
