Well, you've found a good LFS, then! *thumbsup2 A lot of them keep puffers in freshwater. So now - you need to know what the salinity they are being kept in at the LFS and build your tank up to that. Or - if you want a shortcut, you can use 1/2 the amount of marine biospira. That's what I did when I converted my 55 gallon to brackish, I bought some marine biospira and instantly converted my tank so it was able to handle my brackish water & figure 8 puffers It wasn't that expensive, I think it only cost like 15 bucks.
A good thing about figure 8 puffers is that they are used to fluctuating salinity due to their native waters (river estuary). I fluctuate mine from almost freshwater to 1.015 salinity. The majority of the time they are in 1.008 salinity, but I mix it up a bit, too a few times a year....and they adjust fine...it's what they deal with in their native waters.
A good thing about figure 8 puffers is that they are used to fluctuating salinity due to their native waters (river estuary). I fluctuate mine from almost freshwater to 1.015 salinity. The majority of the time they are in 1.008 salinity, but I mix it up a bit, too a few times a year....and they adjust fine...it's what they deal with in their native waters.