Q & A from another site..cut and paste..
Puffers: Hard-Shelled Foods Keep Their Teeth Short
Q. I have a Spotted Puffer in my 75-gallon fish-only saltwater aquarium. It seems that the puffer's teeth have grown to the point where the puffer is unable to close his mouth, which has made it difficult for the fish to eat. Is there anything that I can do to correct this condition?
A. Puffer fish, in the wild, feed on many different types of hard-shelled invertebrates, hard corals, and even coralline algae. In order to keep up with the abuse of this menu, their teeth are constantly growing. If the puffer fish in an aquarium are not given a diet that contains these types of hard foods, their teeth will continue to grow to the point where the fish are unable to close their mouth. This situation makes it very hard for the fish to feed.
For regular feeding, feed the puffer hard-shelled invertebrates, such as frozen foods like whole cockles in the shell, and a variety of mollusks from the seafood counter. These foods will help keep the puffer's teeth short, while providing the necessary nutrition. It is best to offer these foods to the puffers at least twice daily.
If in the event, the puffer's teeth have grown to the point where the fish is unable to close its mouth, more drastic measures must be taken. It may become necessary to file the teeth down. This must be done with great care and gentle handling. If the puffer inflates, it could result in serious health problems. Consult with your aquatic veterinarian or other professional if you think your puffer's teeth are overgrown.