Salt is used to help cure many diseases and parasites, particularly ich. Other than that there is no reason to add salt, so as Blinky asks, why do you want to add it. Freshwater fish are just that freshwater. In the wild they do not live in salt and even if they can tolerate it why do it to them. Adding salt as a "tonic" is like taking antibiotics everyday just in case you catch a disease. It just isn't needed.
As to whether or not it will hurt scaless fish? Over the long run salt will indeed hurt scaless fish, but when used as a short term medication (3-4 weeks) it should do no harm in low doses (1 tsp per gallon is considered low).