I recall a time when I tested a water sample with a refractometer (the instrument was calibrated weekly with 53.0 mS/1.0264 SG
calibration fluid) and it registered a reading of 1.016 SG
...a swing arm hydrometer measured the same water at 1.022 SG
. Prior to the refractometer's reading, the owner of this water sample and hydrometer couldn't fathom why the only corals that didn't die in his tank were mushrooms.