Most people think of sodium chloride when they hear the term salt. To the chemist salt can be potassium chloride, Calcium nitrate, magnesium sulfate, potassium sulfate and many many other chemicals. They are all very different, all called salts, all are naturally occurring, and many are used in plant fertilizers.
In Chemistry Salt is the byproduct of a reaction between an acid and a base. So when you read about salt in an aquarium ask yourself what type of salt they are referring to and what they are trying to do. Ordinary table salt often has no beneficial effect for fresh water fish.