Mystery magnesium

[asylumdown]

SO this tank has been set up since April. So far, I've not dosed a single drop of magnesium, and yet I've never had a test result come back below 1400ppm, averages around 1440. I thought it was my test kit, so I bought a new one and same result.

I'm dosing arm and hammer baking soda for alk, and was previously dosing a bulk anhydrous calcium chloride solution but recently switched to Kent turbo calcium (effectively the same thing) because I ran out. Is there magnesium in either of those two things? i'm also using H2Ocean salt, which I find to be pretty high in magnesium to start with, but before and after water changes my levels bump from the 1440 range to the 1480, 1500 range, and then barely fall between water changes.

Any ideas?