What kind of fish do you have? Most fish do not need the salt, and its better to keep the water completely freshwater to prevent the adaptations of parasites...salt resistant strains can be devastating to not only your fish, but others too.
Salt CAN be used as a natural fix-all (or almost all
) for most diseases and is a good tool when you aren't quite sure what is wrong. It also alleviates gill stress during a nitrite spike from cycling. You do have to be careful when using salt even short term though.. salt can be detrimental for many types of fish.