Some hobbyist have implied this, but I've not read where they offered any quantification for support. Eg, drops from say 1300 to 1100 ppm.
I suppose if you never do water changes, then sure.
Then you need to follow up and do the dry weight tissue analysis for leaves to see if the mangrove kept this Mg in the leaf tissue(or root, or trunk etc)
Otherwise, you cannot say much. A little correlation, but not much else.
If hobbyists say/suggest it drops, they need to say HOW MUCH.
Randy did, but not directly, still, the excretion suggest it's not a way to remove a lot of Mg from the tank, this means the plant itself would have to accumulate and store a lot of Mg, which is not exactly in the plant's best interest.
http://www.reefs.org/forums/topic2750.html
I've done ample dry weight tissue analysis and fertilization on aquatic plants for a long time now.
While useful for some things in aquariums, I think just plain decent common sense things like dosing, water changes, good horticulture etc, cannot be over emphasized. I suppose you could measure and dose say 10 ppm a week of Mg, no way that any plant would require this much for growth, maybe if you stuffed a massive tree in a tiny tank, but the relative biomass to most reefs and marine tank's volumes are very small to the mangroves kept.
Your Byropsis might die back though.
Still, it'd take some time to deplete 100 ppm worth of Mg.
Hopefully most would do a few partial water changes over 10-20 weeks.
I think you'd have to try in most cases to deplete the Mg and pack a lot of trees into a small space.
I think using say aquarium water from a freshwater planted tank or fish only tank and spraying the mangroves every day would help. I do this for Bonsai trees as well.