I have not tested my new makeup water for Mg yet, but I have been using RBS also for about 4 months. I took a leap of faith and didn't test levels - which was a mistake. (I always test Ca and Alk every week, so Mg falls off my radar easily - because otherwise for multiple tanks I sit there and test for an hour which .. well, it's not a good excuse, but it is my excuse).
Anyhow.
Mg among 3 reef tanks was all in the 900-1000 range when I started testing Mg again. So I don't know what I'll find when I go to test my makeup water, but I'm not expecting to find it's very high.
I'll post back #'s if I do get around to testing a new batch of SW makeup water.. Now I'm catching up on all the Mg tests I didn't do because I'm testing every day whilst I try to raise Mg back to seawater levels .. ugh! Lesson learned, don't slip on testing if it's a parameter that's important to you.
