Thanks, everyone, for your kind comments
After a very sleepless night, I found the tank to be in slightly better condition this morning. The two remaining fish are alive and swimming about, though not as they had been with their other two fish friends
Shrooms are enlarging a bit, which I take as a good sign. My favourite macroalgae isn't doing very well, sort of seems limp. Other macroalgae seems fine, but who can really tell since it's a coarse brown variety.
After lots of thought, I'm guessing the black stuff in the foams was dead cyano, which may have accounted for the rise in ammonia.
I may have unleashed a sort of red tide by dislodging the cyano from the tank wall and into the water column, but this is only speculation. Still have this cyano on the bottom of the tank in some places, as well as on some of the rock. The 120g has about 150 lbs of LR.
Only began to notice the cyano growth after using a UV sterilizer to get rid of what we thought was ich on the chevron butterfly. Turns out the fish had a viral infection that was cured by increased tank temps from 77F to 82F for about 3 weeks. Maybe the cyano growth was fueled more by the temp increase than the UV, though cyano growth was at its greatest where the water came out of the UV hose.
A few snails were found on their backs this morning. Put them at the front of the tank and turned them over to watch if they move away or if they are dead.
Still feel really freaking bad about the whole incident, which may not be over for several days as livestock begins to either flourish again or die slowly
Dang
This is not the first major disaster in my reefing career, but that doesn't make this one any easier to handle than the others