Just to add what I know about water turtles; most are not really that social and are usually solitary unless in breeding season so there's no real advantage to personifying a turtle's needs. Males can be aggressive toward both other males as well as females when they are in season. Even two females can have issues. As mentioned, females can get quite large and you really should provide as much space as you can. Also, the bigger they get, the more quickly the water can be fouled, so more area is better all the way around. A huge reason I built my pond is because my two painteds (both were about 5") needed more space. I went from a kiddie pool on the driveway to a full fledged pond. If you have room and desire, I'd suggest you put in a pond of your own; you'll enjoy it and so will your yellow-belly. Marshall hopefully will chime in as he IS our resident expert when it comes to water turtles.
Oh, and don't put any fish with the turtle unless you're prepared to see them as meals. Especially in a small area as the fish will probably not escape. I've found my painteds do okay as long as I keep them well fed and the fish are large. Once, long a go when I was less educated re turtles, I put in a frog and had to quickly rescue him when the turtles when after it. With enough room and space, an equilibrium can be had but an aquarium isn't the way to do that, but a pond is.
Hope this helps.
Michael
