Brackish fish can tolerate FW for some time, but in the long run, health problems will arise. You should NOT attempt to convert freshwater fish to brackish--gouramies are not brackish, and while they too can tolerate brackish for a while, in the long run you are killing the fish.
'Freshwater salt' in no way mimics brackish or salt water.
'Freshwater salt' in no way mimics brackish or salt water.