As both MWood and FHM have said, salt is not a good thing to use routinely in a tank. Table salt, the same thing as aquarium salt, can be used as a treatment for ich but is not a preventive. Sea salt can be used for brackish water fish as long as you have no other fish in the tank except brackish fish. If you have brackish fish, it is a commitment to a tank being nothing but brackish. Either kind of salt will kill the typical aquarium plants and can harm sensitive fish. There are still some people around who would advocate using salt indiscriminately on mollies but what mollies really need is good water that is high in pH and is high in mineral content. I have water like that but most people would call it hard water, not salt water.