If a puffer doesn't eat things with a hard shell, the "beak" (or teeth) will continue to grow to a point where the fish cannot eat, which will lead to them starving to death (obviously). In the wild, these fish eat shrimp, crabs, clams, and other inverts with shells. Look on a popular video sharing site (You something....do you get what I mean?????) and enter "puffer eat" or "puffer eat clam" to see what this will look like, it is actually very neat to watch!