Raising Mg with Kent Tech M specifically is what works, not just raising Mg. It's believed to be a byproduct of the supplement and not the Mg itself that kills the bryopsis. You want to dose the product a bit daily (maybe 20 - 50 ppm) and the daily dose of this byproduct is what will wipe out your bryopsis while also raising your Mg.
Worked perfectly for me, but there are always some tanks that it doesn't work on.