try blood worms, that's about all they'll eat. You can't feed just snails though, they're not very nutricious. Ration the snails, keep them in another tank or container. Otherwise the puffers will eat them all until they're gone. Also, don't feed malaysian trumpet snails, only pond or ramshorn snails. The MT snail's shell is too hard for puffers and can even crack or break their beak.