Why is my Mg always so low?

[c_scherer123]

I have had my 20g for about 8-9 months. I have perpetually low Mg, even though I dose it bi-weekly or more often. The fresh salt batches come in at 1400 mg.
Calcium is 500
Magnesium is 900

I can tell when my magnesium is below 1200, as all my polyps (especially the GSP) and torch corals close up.
I did notice that the coraline algae is starting to grow all over the rocks and glass. I removed the small bit of chaeto I had and that noticeably helped keep the MG level steadier.

Any ideas?