BS: You've unintentionally confused the issue.
Just to clarify:
In a freshwater tank, there's no need to add aquarium salt, or any kind of salt, unless you are treating ich. If this is the case, you add the dose of salt gradually, mixing with some tank water. Avoid dropping large amounts of salt directly into your tank, as you may 'burn' any fish that swim through it, and adding it in portions -- perhaps a third at a time -- will allow the fish to get used to it gradually.
In a brackish tank, you don't use aquarium salt, you use reef salt mix. Aquarium salt cannot make brackish water or saltwater as in seawater. The two are very different. In this case you mix water to a predetermined salinity using a hygrometer to measure salinity, then add to your tank. Were you to add salt mix directly to your tank, you would abruptly change the salinity and stress your fish.