Tell me about magnesium!

[Aquattro]

Tony, that explains a lot, thanks. I suppose with my tank still in it's infancy, things might change as the corals take up more Ca/alk, but I've never measured and needed to add. I'll watch it now that I have a test kit, and perhaps I'll need to add some media, as I'm guessing having a large reactor has so far spared me from this stuff.
And ok, 47 tonnes might have been a bit of an exageration

So you aim for higher than 1350ish?? Like what, 1500ppm?