A few Fish are Intolerant of Salt. Corydoras catfish and many live aquatic plants do not do well with salt in their water. Salt lessens the Number of Pathogens in the water. For example very few, perhaps only one or two groups, of freshwater bacteria tolerate salt well. My experience is that salt is slightly effective in preventing fish trouble in freshwater aquariums but only slightly.
Salt is really most Effective, when the concentration of salt changes.
So I keep my freshwater fish in water with very little salt. Then when I see one or more of the Signs of Stress or Disease, such as clamped fins, I immediately add 1 Tablespoon of Aquarium Salt per each 5 gallons of aquarium water. I also add 1 drop per gallon of Quick Cure, and this combination usually cures fish that have just started to show Signs of Stress and Disease. Click here to read more about the Signs of Stress and Disease. Marine Aquarists often give their marine fish a freshwater bath, and this is the same principle of changing the concentration of salt, but in this case by lowering rather than increasing the concentration of salt.