i would go with a couple red sliders, if you can surround it with a small fence just to be safe that they won't crawl out
maybe add a pleco, you could also do a couple natives, like bluegills, rockbass. let me know im actually raising a bluegill just to see how it turns out. and i've converted him to pellets in like 2 days. if you want next time i go fishing i could keep a couple for you, if you like.