For reference,
natural ocean water has a salinity of about 35 ppt, corresponding to a specific gravity of about 1.0264 and a conductivity of 53 mS/cm.
As far as I know, there is little real evidence that keeping a coral reef aquarium at anything other than natural levels is preferable. It appears to be common practice to keep marine fish, and in many cases reef aquaria, at somewhat lower than natural salinity levels. This practice stems, at least in part, from the belief that fish are less stressed at reduced salinity. Substantial misunderstandings also arise among aquarists as to
how specific gravity really relates to salinity, especially considering temperature effects.
Ron Shimek has discussed salinity on natural reefs in a
previous article. His recommendation, and mine as well, is to maintain salinity at a natural level. If the organisms in the aquarium are from brackish environments with lower salinity, or from the Red Sea with higher salinity, selecting something other than 35 ppt may make good sense.
Otherwise, I suggest targeting a salinity of 35 ppt (specific gravity = 1.0264; conductivity = 53 mS/cm).
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