When you calibrate a refractometer, you should use 35ppt calibration solution for the most accurate reading. Others advise to use RO/DI water, which is incorrect and will lead to misleading values. AccuraSea (get it?) is designed for the hobbyist.  Place a few drops on the clean lens of your refractometer, and adjust the scale to match 35ppt.  That's it, your device is now ready to measure the salinity of your aquarium or newly mixed saltwater. 

Do you think it’s important to maintain salinity precisely? We’re with you. That’s why we created AccuraSea Seawater Reference Solution, the only calibration solution of its kind made from real seawater. It’s pure natural sea water micron filtered and UV sterilized then adjusted with reverse osmosis water to the standard S = 35 or conductivity of 53 mS/cm at 25°C (77°F). Use it to accurately calibrate seawater refractometers and check the accuracy of swing-arm hydrometers. You may also want to use it as a reference for test kits used to measure the concentrations of various seawater components, since it is natural seawater with the correct and well-known concentrations of all dissolved salts. Packaged in 250 ml bottles.