I had previously dosed either Tech M, Reef Complete, or mixed up some BRS Mag. I never got into mixing 10:1 Chloride to sulfate (or vice versa

). My mag always seemed to be in the 1300's. But, I was consistently doing 40Gallon water changes with Reef Crystals. I used to test new batches and it was pretty consistently around that 1300.
Once we decided to sell and the baby came along, my discipline suffered greatly. I haven't tested Mag in about 6 months, and will get all new test kits for the new house. It's been so long, I won't count on them even though they might be perfectly fine.
Once your system grows out the way yours has it definitely becomes a Ca, Alk, and Mag SPONGE! If you're not consistent with your waterchanges or you change salt, or alter the system it really can go haywire like a spinning top right before it topples over.
If you look far back enough, RandyO states that ominous word "Kaboom" for SPS dominated systems. Eventually something's got to give. As the size of your corals grow tho the freakish size yours have, intuitively, the volume of water they are in, becomes less. It's an inverse relationship that favors mayhem. Demand goes up, more toxin in a diminishing body of water.
Most of the long lasting stony reefs I've seen, i.e. several TOTM's, require almost constant pruning.
I've obviously gone into this way beyond the scope of your question, but, when you begin to have swings in your system, you need to be on your toes.
Bring a sample of water to ABC, or use one of Jay's test kits or something. You wanna be sure because the alternative is a surplus of white coral skeletons.
A low Mag can create falsely elevated Ca and altered Alk readings. If you get a solid read on your Mag and it's low, you can bet your Ca is too low too, and that your Alk is not reliable either.
I'm blathering on now......:mtool: