Testing salt water with a refractometer for the proper specific gravity or salinity is the easiest test for aquariums. Most Refractometers do not read accurately when calibrated at -0- with RO, RO-DI or distilled water. Calibrate the Refractometer at 35 ppt. Use a precisely formulated scientific standard calibration solution. Solutions purchased online from reputable aquarium supply company will work. In some cases a Refractometer can be off by .001-.004 points when calibrated improperly, this alone is enough to cause major problems with aquarium inhabitants.