I also have kept a few different species of puffers, and by the looks of things those GSP are still quite young which is why they could be in Freshwater (although still no recommended by any means)... but as they grow they will eventually need very high end brackish almost full marine for you to keep them alive...
Also your tank looks pretty bare, so i'd suggest that you get some decor in their ASAP, since puffers are known to get bored very easily in a bare tank, not to mention aggression gets a lot worst when they don't have anything to break their line of sight...