Like I said I strongly advise against using salt for FW fish. The way i see it is that it's better for the fish to develop a healthy immune system to be able to fight off diseases naturally. In the event that the fish gets sick salt can be used as a treatment for a boost. When salt is used all the time the immune system is weakened, and when the fish does get sick salt will not be a suitable treatment. Salt is not natural for FW fish as they will not come across it in the wild.
Prevention is better than cure as mentioned. The best prevention I can think of is good tank maintanence, proper temperature and suitable tank size/tank mates, basically just providing the best environment you can for your fish. Not the addition of salt or any other substances that the fish was not naturally built to handle.