Welcome!
IMO, you should get at least a 125 gallon for a small shoal of Red Bellies. By small shoal, I mean maybe starting out with 8 to 10 small ones and ending up with 6 or 7. Red Bellies are not picky eaters, and should be fed a varied diet including pellets, fish fillet, shrimp, krill, and etc... Red Bellies are messy, and I would filter a 125 with 2 AC110s (for mechanical) and an XP3 (for bio).