Here are some facts about aquarium salt:
It helps to prevent disease. One of my most recommended treatments for sick fish is a salt bath. A salt bath for 15 minutes, with one teaspoon of salt for every gallon, can stop ick, flukes, parasites, and help to treat bacterial infections.
It promotes healthy gill functions. Aquarium salt contains needed electrolytes to help fish absorb oxygen and expel carbon dioxide. 1 tablespoon per 5 gallons or a half teaspoon per gallon should be used in the main tank.
It doesn't evaporate. Be sure not to add more salt after topping off a tank. Only add salt after a water change, or when setting up a new tank.
It's dangerous to plants. Be careful adding salt to a planted aquarium, as this could damage, and even kill your plants.