Kurt got it.
Our refractometer are made to measure saltwater not seawater, which is what's in our tanks. The fliud Kurt linked to is a seawater calibration solution and it's what you should use to accurately calibrate your refracto for measuring seawater.
Refractometers and Salinity Measurement by Randy Holmes-Farley - Reefkeeping.com
"Fortunately for aquarists, the differences between a salt refractometer and a seawater refractometer are not too large. A 35 ppt sodium chloride solution (3.5 weight percent sodium chloride in water) has the same refractive index
as a 33.3 ppt seawater solution, so the error in using a perfectly calibrated salt refractometer is about 1.7 ppt, or 5% of the total salinity. This error is significant, in my opinion, but not usually enough to cause a reef aquarium to fail
, assuming the aquarist has targeted an appropriate salinity in the first place. "