Is the puffer in a
fw tank? With other fish?! Eeek. If they are you better get him out they're aggressive and are born in
fw but by the time they are a few years old they should be in
sw.. So by the looks of it your puffer should be in brackish atm as your slowly killing him

Puffers can just 'snap' one night.. By the day he could still be happy and by night he could have fins sticking out of his mouth. I'm not trying to sound mean I just want what's best for your fish (P.S. if he's got his own brackish tank I'm sorry for the lecture)