this topic should be in the brackish forum. But I cant answer your question as it depends on what salinty the fish requires. Just because a fish is labeled "brackish" doesnt mean all brackish fish require the same salinties and live in the same tank. Things like figure 8 puffers, knight gobies, bumble bee gobies and many others require water with a SG of 1.005 or lower. Things like scats and monos require an SG of 1.010 and higher. The fish at 1.005 in FW would have no negative effects as long as the the water is hard and alkeline. These fish (the fish at 1.005 and lower) often live in FW as often as they live in brackish in the wild, but in home aquariums they seem to do best in slightly brackish water. The fish at 1.010 would have a very short life. But in particular, glassfish do very well in FW, because unlike most lightly "brackish fish" these fish inhabit acidic FW and brackish, while the rest inhabit hard alkeline Fw and lightly brackish. So at acidic FW, hard alkeline FW, and lightly brackish, glassfish do well.
