Trouble keeping Magnesium up

So over the last week or so, I've been dosing heavily to get my tank to where it needs to so that I can figure out how to set my dosing pumps...and to my surprise, the most difficult element to maintain has been Magnesium.  I'm using the BRS magnesium solution made up of 5 cups of Magnesium Chloride and 3 cups of Magnesium Sulfate per gallon of RO/DI water. 
My target parameters are:
  Alkalinity - 9 dKH
  Calcium - 420 ppm
  Magnesium - 1350 ppm
Over the past four days, I've added 3000 ml of magnesium solution and have gone from just 1260 ppm to 1350 ppm.  I just don't understand how it can be using so much magnesium.  I thought that it was calcium and carbonate that were used by the corals and the magnesium was just used to buffer...not consumed.  So why am I losing so much magnesium every day?
In comparison, to keep my calcium and alkalinity up, I'm only using something like 115 ml alkalinity solution and 20 ml calcium solution.  Any ideas why I'm having so much issues keeping my magnesium in check?
--AJ

 

A little bit of reading has told me that the biggest consumer of magnesium is coralline algae...and that if your rocks don't have a nice layer of purple on them already, that growing the coralline on these rocks is going to suck a large amount of mag out of the water. So I guess I'm in for a massive coralline explosion or something....at least that would be an explanation...but until then, at around $10 a gallon, it's going to be expensive to keep it in check. Maybe I'll switch to Randy Holmes-Farley's DIY recipe vs. BRS's.
--AJ