[Delphinus]

I've something along the lines that higher Mg can impede the growth of certain algaes like Bryopsis, but I was under the impression that Mg had to be pretty low, like maybe <1000 or something. Natural seawater is supposedly somewhere in the 1250 ballpark so I think 1300 is fine. I tend to target my tank to have Mg around 1450 but I do this for 2 reasons unrelated to algae - 1) I can't definitely explain how or why, but my anemones just look better and more inflated and so on when Mg is at 1400 then when it is at, say, 1100 or 1200. In fact, if I notice my anemones looking less inflated I immediately check Mg and so far 100% of the times this has happened I've found Mg to be low (this was before I would adjust my water change water to have higher Mg, this way when I do a waterchange it bumps Mg back up and I never need to dose the tank itself anymore). 2) Since Mg will deplete over time at a rate proportional to Ca and Alk, having it higher than needed allows for it to deplete without it getting too low in between waterchanges. It seems things are more tolerant of Mg being slightly too high than slightly too low (if that makes any sense).

Anyhow long story short, at 1300 I wouldn't sweat the Mg value but just maybe add a little to your water change water so it goes up to say 1400 after that and you should be set.

Otherwise, stay the course, sounds like you have a good plan for things to settle out. Good luck!