well my guess is along with alk and calc the blue bucket is also on the lower side of the trace elements. i did use the blue regular red sea salt for a while and actually like that too ..Use red sea blue bucket if you want low 8s Alk. I am dosing 60ml ESV Ca and Alk daily and am stable at 420 / 7.6, and I dose 6ml ESV Mg daily and it stays in low 1300s.
I'm in the same boat, looks just like my readings, I just mixed a fresh gallon of mag, it's funny this topic can upI dose 10ml every day to balance the CAL and ALK from Calcium Reactor. Always stay at 1250 - 1350...Cal betweeen 420-450 and ALK between 8-9.
As a chemist by trade, yes I certainly do enjoy testing!Time for the contrarian view
Since you are keeping Calcium and Alkalinity well in balance, I don't think you need to check Magnesium unless you really enjoy testing and dosing, and want another number to chase. There is 3-5 times as much Mg in water as there is Ca; it gets replaced by water changes and is slowly taken up by corals. For infrequent water-changers, probably a different story.
As you can see by reading the thread, I'm in the minority here. But there are people out there who never test Magnesium and still keep SPS successfully.