I bought a refractometer a few months ago and since have lost the instructions for it. Now I have my tank up and running and took a H2O sample to my LFS
to have them test it. Their SG
reading was 1.021, but my refractometer is reading 1.025. Can any one give me info on how to go about checking the calibration? I saw a post that said to use RO
H2O but what should the reading be?