I keep mine at 35 ppt.
Ron Shimek's Website...Critters
"Coral reefs are generally located in areas that have salinities in the range of 35 ppt (1.026) to 38 ppt (1.029). Most of our corals, and the associated fauna including fishes, will live best at those conditions (Weber and White 1976)."
"The bottom line for salinities is simple. There is simply no reason at all to maintain the salinities of our systems below normal reef conditions. All reef inhabitants will suffer damage from prolonged exposure to lowered salinities. Invertebrates kept at low salinities often die within a few days to a few months. Given that corals, sea anemones, sponges and some other invertebrates have no old age or senescence (or to put it another way, they are immortal), low salinities result in a quick death. Some mollusks, crustaceans, and most fish kept at low salinities die of kidney failure; it just takes them longer. A fish which dies in a couple of years in a hyposaline aquarium may have had the potential to live more than 20 years had the salinity been appropriate."