Actually, you can, but if it concerns you, purchase kosher or pickling salt, which is uniodised.
I (and thousands of others) have been using grocery store grade (which is very pure) salt and baking soda with our fish (including African cichlids, which I have been keeping for 27 years now) for decades with great success. If you really wish, you can puchase laboratory-grade from a lab supply, but expect to pay quite a bit more....like $50 vs $4.