CA/ALK/PH and MG questions

[fresh]

Hi all,

I have been using the home-made 2 part CA and ALK system for a while now and have noticed lately that my ALK always drops faster than my CA!

In the span of a month, my CA went from 400 to 380 while my ALK went from 9.2 to 6.4.

I haven't started adding the third part to the equation and that is the magnesium sulfate.

I have already added the ALK once by itself without adding any CA hoping that would fix it, but it didn't.

The questions that I have are:
1- Is the CA drop normal for an average stocked sps/clams tank in 1 month?
2- Is the ALK drop normal for that tank in 1 month?
3- Is not adding the magnesium sulfate the reason for the ALK swing?
4- What can I do to balance out the equation and where am I going wrong?
5- How bad is ALK 6.4 to my corals? Do I need to panic?

I have a 72gal tank with low bio-load, 100+lbs of lr, 4" aragonite dsp, and a good skimmer.

I have a few good sized sps colonies, 2 large clams, a couple of large lps, and numerous frags of sps.

Thanks for taking the time to read, and I appreciate all the help I can get.

Fresh.