Is it that you need salt for some non-aquarium purpose and are thinking of using aquarium salt, or that you need a 55g drum of aquariums salt for aquarium purposes?
Aquarium salt and food grade (I.E. table salt) shares the same main ingredient, sodium chloride, but aquarium salt contains a whole bunch of other stuff such as trace elements, and table salt contains one big ingredient that aquarium salt doesn't.
I doubt it would hurt you to use aquarium salt in food preparation as a one-off, but it wouldn't taste very good. Table salt contains potassium iodide (or something similar) to help prevent thyroid defficiency, so using table salt as aquarium salt is a very bad idea.