Making a pond from a swimming pool ? ?

I have known it done where a swimming pool was turned into a small lake for people to fish from. I only knew it as the fishing lake but was told the history by some of the other people that fished there. It had been quite well landscaped so it did not appear square or anything and the only real give away was the depth. When I was fishing it there were carp over 20lb and plenty of other smaller species. Being a fishing lake it was not filtered or anything and seemed fine so obviously it can be done. I think domestic fish may need the filtration but that is easy enough to do.