Low magnesium

[Treebeard]

I just recently started testing magnesium and tests on consecutive weeks indicate Mg at 1050. Calcium is consistently 425 and alkalinity ranges from 8.5-9.5. Ph has consistently been on the low side ranging from 8.0 to 8.2.
I am using kalkwasser in my ATO and do weekly 20-30% water changes using Reef Crystals.
I do not have a lot of SPS, but do have lots of LPS and a couple of big clams.
Why would my Mg be so low? What is a good supplement to use to raise it?