Shrimp, mussels and crabs should make up at least half the diet of marine puffer fish. Try to use dead foods rather than live. You can buy bags of mussels, prawns, crabs, etc, from seafood stores or the seafood section at supermarkets. keep them frozen until needed, then defrost one and let the fish destroy it.
You can also collect rocks from the ocean that have mussels and barnacles on them. Put the rock in the tank and let the fish eat what's on it.