I just calibrated mine today (was reading 30ppt with 35 solution). Can the calibration fluid be poor? I figured you can't screw up the fluid, and it gets good reviews. My ppt is at 39 now so I have to lower it with the ATO. Hopefully it doesn't kill anything.As long as the refractormeter is calibrated correctly, they are the same.
Higher quality Refractometer can be calibrated by distilled water, most $20 to $50 ones need to be calibrated with calibration solution, normally at 35 ppt.
This is due to the quality of the optic that affects it linearity throughout the range.
I can calibrate your refractometer for free.