You could try Flag Fin Shiners... is this a ten gallon?
Keep at least three- they like to school. I'm pretty sure they stay smallish-not over four inches long. They dont require a heater,eat anything and are hardy once established. ( and they're pretty too!)
You can probably find them at a bait store if you cant catch your own. They are alot like red tailed sharks in appearance. Only thinner looking/more streamlined.