When I was a kid, I had a box turtle pen in my backyard. It was probably 12' by 12' with a short board fence around it, no more than a foot high. It was low enough that they would get out of it at times and I'd find them in the nearby strawberry patch.

My dad built a tiny pond in it with concrete. It couldn't have been much more than 3' by 2' and no more than a foot deep. I used to throw minnows in there I'd catch from a creek and never remember having a fish kill (except that one time when I added a small snapping turtle to my turtle pen.). I don't think they ever produced young and I never used aeration or filtering and don't even remember feeding them. I also recall a friend of my dad's who kept minnows in his backyard in a small rowboat that filled up with water.