Good question. plus it really helps to know the salinity the water they were in at the LFS.... some keep them pretty salty to help enhance their colours.... thus make them sell.
I have two GSPs that are approx 1.5" and keep them in SG 0.008 they are pretty happy in that..... however did loose some when the water was fresh as I'd been sold them as DPs.
Well, at my LFS they have a liquid that you mix their food in before feeding them. You use the medication for 1 week, I forgot what its called..... You can also get a flake food that is already made to destroy internal parasites.
I believe a lot of people that own Puffers have also used Discomed with success.
Ask your LFS guy and he'll most likely have what you need.