Well, it can't adjust just like that. First of all, they will live shortened lifespans in brackish, and second, it was the change in water that did it. Was it in brackish water at petsmart? No.
they wouldnt sell me my dwarf puffers until i bought aquarium salt....i told the lady they were freshwater. lol i've had them for over a year in fresh water.
ya tiger barbs are fresh water and do much better in groups of four or more. and prolly not the best tankmates for a dragon goby and a silver shark