A UV sterilizer won't help out the fish though....
What else do you have in the tank? What kind of puffer is it? Usually, I would say a cleaner shrimp would do the trick, but I'm wondering if it would just end up as a snack for the puffer.
There are some medications, baths and dips, that could help but let me know what else is in your tank (ideally, you'll want to medicate in a seperate tank) and what kind of puffer it is.