if they are indeed dwarf puffers, then they are totally freshwater and should not be kept in brackish (salt) water. now, it could be that they are actually tiny green spotted puffers (which can get up to 6 inches....), which are brackish and require salt. ask the store if they know the scientific name of the puffer they have there to be sure. if they don't know it, don't buy them until you can positively look at the fish and know for sure which kind of puffer it is.