Frogs and tadpoles, minnows. Properly sized of course. When I kept mine, years ago, I didn't know better and fed them goldfish. They were gluttons, eating 5-6 in a sitting!

If you want to wean him/her onto frozen foods. Go to your local fish store (pet fish, not the seafood market ), and ask about frozen silver sides. You should be able to get them in different sizes, or just cut them into smaller pieces.