He did shout out loud not to use distilled water or ro
water for calibrating refractometer but never explained any reason why. We are hobbyist and not scientist to have a perfectly calibrated salinity tester. I would say the free standing hydrometer is reliable enough as long as you level the table properly where you put it. If you notice the graduation of the refractometer is too close and tiny that you can hardly see a space in between bars. Compare to the free standing hydrometer you can really see more space in between bars. As I mentioned, the good thing about the refractometer is you only need 3 drops of the sample for your test which is very convenient when acclimating live stock.