How much do you normally change once a week? coz thats pretty much the last thing i can think of causing problems with the fish...
Anyway, i'd probably just keep an eye out for him... btw, did you ever treat your puffer for internal parasites when you first got it? since puffers tend to come in with some form of internal parasites and that could be whats causing problems as well...
Lastly, not trying to sound rude here, but a 90G tank for a puffer that will reach around 27+ inches isn't going to last in the long run just so you know...
Anyway, i'd probably just keep an eye out for him... btw, did you ever treat your puffer for internal parasites when you first got it? since puffers tend to come in with some form of internal parasites and that could be whats causing problems as well...
Lastly, not trying to sound rude here, but a 90G tank for a puffer that will reach around 27+ inches isn't going to last in the long run just so you know...