If you do a search on Mg on the board you will see I had the same problem, I bought the sailfert Mg test kit and tested it. I was down about 500ish (I suspect I had a Mg crash and I caught it on the way back up.)
I could never get my Ca over 300 even with a reactor and lime water.
anyways after I descovered the low Mg I bought "seachem reef advantage Magnesium" and started to raise my Mg levels at a rate of 100 ppm/day (I think) I observed that as my Mg levles raised so did my Ca levles.
here was the original thread, what I can add to it is that when I got over 1300ppm I was only then able to have Ca levels over 400.
http://www.canreef.com/phpBB2/viewto...038&highlight=
Steve
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