Testing salt water with refractometers 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.
The refractometers measures the amount of light shift, called the angle of refraction. This refraction is converted to a useful unit of measure. With a saltwater refractometer, the units are specific gravity and salinity.