What can influence a tank's Mg reading

[Canadian]

Originally Posted by RuGlu6

Ah, sorry no IO for me:
To much dirt and still need to supplement, this means more testing, more time and more money involved.

I know what you're talking about with respect to the "dirt". However, IO isn't alone on this problem - I've seen black sediment and particulate in Kent, Instant Ocean, Reef Crystals, and Seachem Reef Salt over the last year or so of switching between salt brands.