So, to follow up on this:
Raised Mg to about 1700-1725 (from 1250), there abouts over 5 days. Held it at 1700 for about 2 weeks
After about 4-5 days at 1700 bryopsis started to gray out and lost most of its biomass. By 7 days, no bryopsis to be seen.
Where the bryopsis was hair algae started popping up (consuming nutrients left behind by fried bryopsis, which I didn't remove, as it kind of just shrank away to nothing.)
Held it there for a week after no bryopsis was seen. About halfway through that week I did some serious weeding, pulling out all the hair algae that popped up an using a turkey baster to blow out the sedement there (incase any bryopsis bits were trying to take shelter in the detritus where they were growing.)
Did my first water change today. About 20%, have another batch of water that will be ready tomorrow, another 20% change. Then I will take a Mg measurement and do another water change in a week or so, depending on the effect of these last two water changes.
Anyway, looks to be totally nuked. Amazing how well that worked. Tank looks awesome.
I will continue to watch for it, and if I see any, I will NOT pick it this time, as that is what started this mess. Probably just remove the rock from my system. I will treat again if it becomes systemic.
rob
P.S. as a side note, if it is an impurity that does this, it seems that maybe what is important to remember when treating is the total change in Mg, not the level it is at (although maybe high mg is also necessary for the impurity to be active against bryopsis?)
For example:
If my Mg is high, say 1500 and I dose Tech M to 1650. Probably not going to do too much.
Lets say it is low, 1000 and I raise it to 1500. Lot of Tech M going into the system, lots of the mysterious bryopsis killing impurity, even though it isn't at 1600+
Maybe the 1700 is just a good benchmark, based on what people normally have their Mg at (1200-1300), but what is really important is how much Tech M you are actually adding to your system (net change in Mg conc.)
Thought I would throw that out there, since I haven't heard any discussion mentioning that.