There may be a warning label on the box, stating "non-food grade" or "safe for human consumption." There is food grade refined rock salt available, but if there isn't a food safe label on your box, I would consider using something else, like kosher, for your fish. Is your box of Morton called Ice Cream Salt? If so, that is not a food safe product, according to the Morton website, see their link below. Non-food grade rock salt contains impurities that you don't want in your mouth.
Here's an older thread on the subject, with differing opinions. I think if your salt is not labeled as food safe, then be safe and use an alternative salt variety: