Salt has been used for decades as a general "tonic" for freshwater aquarium fish. There are some benefits that anyone thinking of using it may consider: I've used it for several years with good results. However, certain fish seem to do better with it than others. My Livebearers, for instance do well with it. My Corys seem fine with low doses of a teaspoon or a bit more, in every 5 gallons of my replacement water.
Specific benefits include: Good gill function in water with a trace of salt, a good source of electrolytes that help the gills use oxygen in the tank water, stimulates the production of a healthy "slime coat" that's needed for a healthy immune system. Suppresses the development of parasites in the tank. Most fish pathogens that affect the fishes' skin don't tolerate traces of salt in the water. Salt interferes with the lifecycle of tank parasites.
Granted, there are some fish that don't necessarily benefit from it like: Tetras, Angelfish, Discus, Loaches and Bettas.
But, again, there are some good reasons for routinely using a small amount.
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