Puffers can be very picky about their food. Make sure his food is (shell based) krill, RAW cocktail shrimp, ghost shrimp, etc... I've had Green Spotted puffers for 5 years now and very happy. They like hard water, use aragonite or coral to do this naturally. Marine salt can help too, but make it BRACKISH, not full salt! Consider age of the fish you have and check his TEETH!!! Unfortunately if puffers are not given enough foods with shells on them, their teeth can overgrow, making it impossible for them to get food in their mouth. This can cause slow starvation and much misery for you and your puffer.