is it eating?, whats it eating?
a lot of people try feeding puffers on purely blood worm, especially the shops, they need at least white worms and snails to be kept really healthy, mozzie larvae and guppy fry are enjoyed too. if they are fed well enough and water quality is pristine they will beat any parasites on their own normally.
Mine was skinny initially too, but a pile of love made him healthy until the well deserved EX MISSUS dumped a bottle of detergent into my tanks, :an!gry :an!gry :an!gry :an!gry :an!gry :an!gry :an!gry :an!gry :an!gry
gave him a 5 min bath in avitrol on his way home from the shop. its a good time to treat fish that a lot of people miss, ask the store to do a quick worming while your taking them home many stores will be more than happy to help you.
easy enough to set up a white worm, black worm, maggot, guppy fry, snail culture, takes about 10 mins to do per week.