I always calibrate my refractometer with Distilled Water until I almost killed all my SPS as I can't figure what's wrong with my tank as all parameters seems to be right. After some googling I came across this blog:
http://blog.captive-aquatics.com/cap...sing-rodi.html
So I decided to buy a Pinpoint refractometer calibration fluid. I was shocked more than my corals about what I found out. I was running my tank salinity @ 1.030 for the longest time.
I then check other forums regarding this and there are lots of threads with this discussion.