Aquarium salt isn't used for marine or brackish water tanks. It doesn't have the right consistency for seawater; you need marine salt for that.
In fact, aquarium salt is not required for anything; it's a confidence trick based on the old con of putting a special use in front of the name of something completely ordinary to multiply the price by 10, a bunch of myths about iodine and anti-caking agents, and some attempts to confuse with fuzzy science using phrases like "essential physiological ions". Yup. Sodium and chloride, as found in - erm - table salt...
