Ok sorry, I meant mix the salt itself, not the water with salt in it. I once complained about another premium salt brand that was reputed to have Mg at 1400, but when I tested Mg, it was 900. (I'm actually really starting to wonder if the problem is Elos can't tell me if Mg is <900 - it clamps at 900 - just on account I've never had a reading less than 900). Anyhow, I ranted about it, and one thing that came out of those conversations was that you're supposed to mix salt before you use it. For real. Isn't that the lamest thing? But I guess if they're adding this and that when they're making the salt up, the proportions may be all correct as far as the total batch is concerned but the dispersal may not be as homogenous as one would like.
Tank with the flatlined Mg is not new and has no sand. It has maybe a half dozen 1" SPS frags, two smaller colonies, a bubble coral, a gigantea carpet and two ocellaris. Maybe about 3 billion aiptasia though. It's a 24x24x12 shallow lagoonal thing.
Ca is 360, Alk is 7 and Mg is 900.
The other tanks have similar but different readings. One has Ca 420, Alk 7 and Mg 1100 (as of last night anyhow, I am still raising it though), the other has Ca 370, Alk 7, and Mg 1050 (also still being raised). One other is a BB tank, and the last one has a 1" skiff of sand.
I am betting dollars to donuts now that the issue is in fact the test kit can't tell me a number less than 900. I will test tonight with Salifert and see what shakes out.