The salinity should actually go up due to evaporation. Salt doesn't evaporate so the salt level would be more concentrate. To offset the evaporation you might consider installing an auto top off system. You can use some DIY on one, Mine is basically a container for water, an airline hose, a MJ1200 and a double float switch in the sump: http://www.autotopoff.com/products/DS1/index.htm