Thats interesting, I've never heard anyone actually suggest using aquarium salt to create a brackish environment. For what its worth, if you're planning on doing a brackish tank I would definitely suggest investing in a salt mix like instant ocean and NOT using aquarium salt.
As for the whole table salt vs aquarium salt...when you're using such a tiny amount (like when you're using it as a preventative additive to your tank) I would not worry about using table salt. IF you were to use a large amount like is suggested to create a brackish environment I would think that the iodide and silicates would create the same issues of algae etc as they do in excess in a SW tank.
As for the whole table salt vs aquarium salt...when you're using such a tiny amount (like when you're using it as a preventative additive to your tank) I would not worry about using table salt. IF you were to use a large amount like is suggested to create a brackish environment I would think that the iodide and silicates would create the same issues of algae etc as they do in excess in a SW tank.