I did a little research and it appears that it is OK to put marbles in the tank, just make sure you bought them from the store [no "used" marbles!] I would also recommend disinfecting/sterilizing them, as marbles tend to come in a netting and are exposed to all the other customers in that particular store. If you can't find marbles at a pet/fish store, you may want to try an artsy store like Michael's Crafts [look in the floral design section for various sizes, shapes, and colors of marbles!]

 No because your frog might swallow the marbles that you put in