"The lower salinity limit that most reef animals can tolerate at for at least a few hours is about 30 ppt. This represents the salinity normally found in estuaries, or around river mouths or, periodically, in some lagoons after substantial rainfall. Coral reefs are generally located in areas that have salinities in the range of 35 ppt to 38 ppt. Most of our corals, and the associated fauna including fishes, will live best at those conditions."
Comes from an excellent article on salinity and temperature here...
http://www.ronshimek.com/salinity_temperature.html
Check the accuracy of what you are using to measure and know that temperature can affect (in a small way) some forms of measuring salinity...