The salt being sold in stores is more often used for fish tanks, not reef tanks. So it is marked for those people that have a predatory fish only system, with lion fish, groupers, puffers, etc. Those tanks tend to run at 1.021 sg.
Reef tanks run at 1.026 sg / 35ppt - that is what corals are used to and thus we try to match it. So rather that worry about the label on the bag or bucket, mix it up to what your livestock needs. Start out by mixing 1/2 cup salt per gallon of water, and add a little more if necessary.
This article should help:
http://reefkeeping.com/issues/2007-11/nftt/index.php
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