I have been looking at books on in ground swimming pools and on their construction along with that I have even looked at the possible idea of buying a house with a in ground concrete pool. The things I have pictured from the books is that some of these swimming pools and large fish tanks are built almost the same way and expect one of them has viewing windows and a more stronger frame to hold in the viewing windows. What I would like to do one day to a old swimming pool is dig out the side of it and build a indoor room along the side of it and make the pool indoors and build viewing windows along the side of the old swimming pool by knocking down one of it's old walls. That way I could turn the old swimming pool into a giant fish tank and convert the pump and drain system to help with the movement of water in the giant fish tank.
I have an in ground swimming pool right now. 20K gallons salt water, big egg shaped sand canister filter with a good old 2 horse pump on it. The pool has two incoming lines, three if I hook up the bottom crawler vacuum. It has two returns. My turn over is probably 1/2 per hour if that. The turn over is just not needed for swimming pools. My point is, I just don't think it's feasible to convert an existing pool to a pond. Also I have learned in my research the best way to build a water tight concrete container is with a one pour method. Ponds, pools, aquariums all need one pour of concrete. That means the floor and all the walls are the same everything. Once you start drilling holes and adding bulkheads, viewing windows, skimmers, lights and so on, leaks are created. Even adding concrete walls to an existing concrete floor wont work in the long run to hold water. That seam will always leak. Anyway great discussion, I think I'm a year away from something like this. I'm really throttling up my concrete research, you may seem me lurking around the DIY area more........ (evil laugh)