As stated, not going to harm the fish in any way. What it will impact is your pocketbook. Instant Ocean for example makes two brands of salt, Instant Ocean Sea Salt (for FO) and Reef Crystals (for Reefs). The Reef Crystals is more expensive because it supposedely contains a more perfect mix of the various trace elements required by corals. Certainly not going to hurt, but why spend the extra money?