your refractor meter measures your salinty, and your specific gravity which is the amount of salt in your water, I beleive most like to keep it around 1.024. As water evaporates your specific gravity will increase because the salt doesnt evaporate, so topping up your tank and consisitant water changes will keep it where you want it to be!
hope that makes some sense.
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65 gallon saltwater tank, 60lbs live sand, 85lbs live rock, 2 ocellaris clowns, spotted goby, Flame hawkfish, scopas tang, numerous corals 
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