How to Test Aquarium Water Salinity. Maintain the right amount of dissolved salts in a salt water aquarium by measuring the amount of dissolved salt, also called the specific gravity, of the salt water as compared to pure water. You must keep the salt levels consistent to preserve the life and health of aquarium inhabitants. There are several tools on the market to help keep an eye on an aquarium's salt level. These tools make testing the water's salinity a quick, uncomplicated process.
Purchase a portable, handheld refractometer from any aquarium outfitter or pet store that deals in aquarium equipment. A refractometer is a device that uses prisms and lenses to test salt water against the refractive index, or the degree the speed of light is reduced when traveling through salt water. Light waves are slowed to varying degrees based upon how much salt is suspended in the water.
Calibrate the refractometer to read for salt according to the instructions on the specific refractometer. Some models require use of a calibration solution that must be purchased in addition to the refractometer. Some models use distilled water. Check to see what your refractometer requires prior to purchasing it. Generally you calibrate a refractometer by placing two to three drops of either solution or distilled water onto the refractometer's meter. You then look through the eye piece while turning a knob or screw on the body of the refractometer until you see the scale inside reads "0."
Place two to three drops of the aquarium's water onto the measuring surface of the refractometer. The measuring surface is usually the refractometer's prism. The refractometer focuses the light passing through the prism and the drops of salt water onto an internal meter or scale.
Look through the eyepiece at the end of the refractometer to see the scale. The salinity, or specific gravity, reading appears as a line or a shadow line across the refractometer's internal scale. The reading is based on how much the light waves slow when they pass through the salt in the water.
Compare the salinity level given by your refractometer with the aquarium's normal salinity level.
Adjust the tank water's salinity accordingly.