Mg 2+ fluctuation

[cment]

I just purchased a new salifert Mg test kit and used it for the first time this morning before lights went on and it read 1650ppm. Obviously, I was very surprised to see it this high. I tested again this evening right before lights went out and it now read 1500ppm. This is a very large swing in an 8 hour photo period. I duplicated exactly what I did the first time, so Im a lil confused as to why this happened. Would Mg swing that much in 8 hours?