The turtle's max size will depend in part on its sex. Female redbellies get larger than males. Still, you're going to need a much larger tank or pond either way. Do you know which species of redbelly it is?
that is tight for this turtle but it will do until you can get something bigger. Red bellies are very active and need lots of room to both swim and bask so in this case, the bigger the better.
Most are feeder goldfish, however this pond is nicknamed 'Thunderdome', I've thrown some naughty green, ablino, ruby red tiger barbs in there!
But she doesn't seem interested. 6 or 7 years ago she would have destroyed them...