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I have large tanks of Corydoras and Livebearers and add a teaspoon or a little more to every 5 gallons of my water change water and my Corys are fine. I change a minimum of half the tank water weekly.
A little standard aquarium salt has been used in freshwater tanks for decades as a general " health tonic" for freshwater fish.
I've used it in my tanks for several years with no problems whatsoever. If you want to know the benefits of adding salt to your freshwater tank, go to: www.bestfish.com/fwsalt