Why does BRS recommend this?

If you have been using a hydrometer and have never tried a refractometer then you should really invest in one.  They use much less water, and can easily be calibrated for accurate repeatable readings.  For the most accurate readings  you will want to calibrate the refractometer to the solution you will be reading, and that is why we always include a bottle of BRS Refractometer Calibration Solution at 35ppt (1.026sg) to keep your reef tanks salinity spot on.

Must have equipment for aquarists! Easily and accurately measure the salinity of your tank. Simple to use and read. This unit automatically compensates for temperature. Salinity Calibration Fluid included. Calibrating your unit ensures correct salinity readings.