Shrimp, Tilapia, Silversides, are your best bet for food. Cats are active at night, so feed in the evening to get the best food response. He will grow quickly, and end up eating everything you currently have him housed with. Freshawter Stingrays are Illegal to own in FL.
Silversides you can get cheap at the bait shop...(bird and 65th has them), Shrimp and Tilapia...supermarket.