The refractometers used for aquariums are to measure salinity of the water. The ones used in brewing measure sugar content, the one I have measures in brix (unit). The sc These refractometers sometimes are auto temperature corrected, meaning the temperature of the sample being measured can have varying temperature, and results will be still accurate. The scale in the refractometer has brix on one side and sg on the other. The sg scale is based of calculation conversion. It's approx 4 times the brix value. There are many calculators online that can determine original sg based on final brix and final sg. I think northern brewer has a good one. That particular calculator also compensates for alcohol present in the sample. I haven't seen the formula, but many ppl swear by it. I'm just letting my batch clear and settle a bit, then I'll test and post results.