I would buy salinity calibration fluid at 35ppt and calibrate your refractometer with that, IMO RO water or sterilised water or any such liquid doesn’t give accurate calibration as it’s set point (ie 0) is too far away from what your asking it to measure.
I’ve found that when my refractometer and conductivity probe don’t match, it’s because of micro bubbles on the probe. I have my probe in front of a powerhead in my sump and that keeps it reading perfect.
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