Hi, I can see a couple of issues here....
F8's really should be in Brackish water and it sounds like you have yours in fresh....that's not going to any good for them in the long term....they will live in fresh, but it will shorten their lifespan.
Secondly, [and I really am no expert on stocking levels, but your tank sounds over-stocked... I have just 3 F8's in a 40 gallon and for me that's going to be pushing it when they mature and maybe get a little more aggressive....what sort of catfish do you have in with them?
Back to your little one though....he could be constipated, you could try feeding him a pea, most Puffers will turn their nose up at it....but mine will attack one pea for a short time and eat a little! Definately try and get some frozen or live blood worm....do you soak your freeze dried ones before feeding him.....if not this could cause him to bloat up.
Lisa x