The vast majority of puffers do not eat commercially prepared fish food. Smaller puffers will enjoy bloodworms, mysis shrimp, blackworms, smaller krill and other similar frozen foods. Larger puffers will enjoy shrimp, crab legs, prawns, large krill, crickets, mussels and other similar foods. Feed the smaller puffers once or maybe twice a day, while the larger puffers should only be fed about three times a week.
Read more at Suite101:
Puffer Fish for Fresh Water Aquariums: Tips for Keeping Puffers in a Home Aquarium http://aquariums.suite101.com/article.cfm/puffer_fish_for_fresh_water_aquariums#ixzz0fm3g6pI9
In the wild Puffers or Pufferfish are predators eating a variety of snails, shellfish, crustaceans, and other fish. In captivity they will eat almost everything that is offered and should be fed a variety of live, frozen and freeze dried meaty foods. Though puffers are fun to feed and will become quite adept at 'begging' for tidbits from their keepers, keep in mind how often to feed and the resultant load on the aquarium. Feed small puffers (under 2") daily, mid sized puffers (2 - 4") every other day, and large puffers (those over 4") can be fed just two or three times a week.
As and interesting note, many puffers in the wild will blow or 'spit' at the substrate to uncover hidden crustaceans or other foods. In an aquarium some keepers have experienced their puffer spitting water at them from the surface of the water in an attempt to earn some tidbits of food! Be careful not to be fooled into overfeeding them.
Puffers have strong teeth that grow throughout their lives. They need to be offered hard shelled live food often to keep their teeth worn down. Acceptable foods include shellfish, crustaceans and hard shelled foods such as snails. If the teeth get too long, they will be unable to eat, requiring the owner to clip the teeth.
Puffers are opportunistic omnivores; they eat most anything and everything. In the wild they feed on all types of invertebrates, algae and carrion. Very important is the inclusion of greenery in their diets; live algae is best, but a prepared green food on the bottom will do. Especially for the Boxfishes, it is imperative to get them feeding ASAP.
They also need hard food materials, to wear their naturally ever-growing "tooth" plates; shrimp, crabs, mussels & the like in the shell.
Feed moderate amounts and distract your puffer so other tankmates get fed. Be leery about feeding puffers to satiation; this makes a big mess and they can grow too big, quickly. Some species get LARGE, longer than your arm.