You never said what it was, some sort of salt? if so, any salt you buy is going to work for keeping sharks, not that there extremely hardy, but mostly any marine salt you buy is going to work out fine + most every salt is safe on inverts.
there I'm sure are lot's of opinions on whether to try and prevent disease or leave well enough alone and only treat when and if you actually get the disease. Mine on this subject is not to, we add so much junk to our tanks as it is.
Good Luck
It's not a good idea to add a medicine to a tank as a preventative. If you continually add a medicine, many organisims can eventually become immune to it, rendering the medicine usless. Much better to keep water quality high and do regular water changes.
HTH