[mike31154]

I use DI water, sometimes even tap water and have seen little to no difference when calibrating my refractometer at the 0 level and that's with 210 TDS tap water. It is of course possible that the 'slope' of your refractometer could be off making the 0 cal inaccurate at the salinity range you like to keep your system at. It most likely boils down to each individual refractometer. All I know is my critters are happy & I've never used calibration fluid to dial in my refractometer. I didn't pay enough for the thing to warrant using lab grade calibration fluid on it. I reckon as soon as you open the calibration fluid bottle or whatever it comes in, it's no longer useful for future use. If I was going to get that precise in calibrating my instrument, I wouldn't use the fluid more than once. I'm more concerned about ensuring my water change water is as close as possible to my system water since I tend to do fewer, larger water changes. This is where I value my refractometer, whether it's off .00? points or not, as long as both readings, make up water & system water are within .002 of each other, I'm good.