What can influence a tank's Mg reading

[Delphinus]

As of this week (finally finished up my KZRB salt) I'm back on IO. Here's what I tested out last night:

SG@1.025, dKH=11, Ca=340, Mg=1110

My lesson is learned though, I'm testing my NSW for the "big 3" everytime I open a new bag or bucket of salt. Sad that I've had to learn this lesson at least 3 times before it sunk in: First with the bad Alk batch of Kent, then Aquamedic salt, most recently KZ Reefer's Best. I don't care what the hype is about any brand of salt - I'm testing it everytime I open the bucket for the first time. Enough is enough, I don't trust any number printed on the side of a box or bucket or bag anymore, I want to see it for myself and adjust accordingly.