there are a few ways to treat ich
NOT in reef tank:
if the puffer ISNT with your reef you can take the easy path and medicate it, however medication sometime destroys the insides of the fish if you do not change the water in about 2 days. As long as he is not in the reef tank you could bring your salinity down to 1.009 which kills the ich.(inverts will die in low salinity) After about 3 weeks ll disease will be gone. Also raise the temperature to 83 which makes the fish "slime" their skin off more. You could also give the puffer a fresh water dip, which is where you put the fish in enough RODI water to swim in for about 30 seconds.
REEF tank:
A freshwater dip would be fine. Rasising the temperature also would be acceptable. You could also move the puffer to a ank w/o coral and treat with medicine.
Just remember if you treat with copper change your water in 2 or 3 days. The copper might leech into the silicon or rock so be careful with adding new animals in the future.