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?

[Madreefer]

I use a product called Asolute Zero driveway de-icer mixed with Epsom Salts. You buy the de-icer in a 50lb bag for around $40 and epsom salts are cheap. Will last you a couple of years. I believe the mixture is 9:1. I add 1/2 cup fer 5G to my top up water. Would be depending on your water volume but thats the amount that works for me.