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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:
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See this list:
http://www.mortonsalt.com/faqs/index....
It is a Morton Salt site, and tells which salt is not safe for food.



