IMO the damage is done and now you're in zombie coral mode. Acros can slowly die over literally months unless fragged, I've witnesses multiple times. My last episode was a Phosphate spike that slowed growth and led to an Alk spike. Once the damage is done corals will react differently and at different times. Some of my acros just looked horrible for a month, then perked back up with little tissue loss. Others looked ok for a few weeks, then started to slowly STN away until I fragged. Now almost all acros have recovered but one of my green millies, a small frag that was in the tank at the time of the spike, had started to STN.
Get a small acro frag, dip (I use Bayer always) and place in tank. You'll probably find it does just fine. You;re at the phase now where it's tempting to change something to stop the death but changes will only put more corals at risk. IMO.
I photo document just about everything so I have pictures.
This is a subulata, I think. This has some minor tissue loss on the most shaded part of the coral but it never spread.
Prior to the phosphate spike, October 2014
Post Spike (looking like it's about to RTN), November 2014
Slow slow recovery, January 2015
This is my Purple Fuzzy, prior to the issue.
Post issue, rapid tip dieback, then slowly STN'ed until I fragged it down to just a single healthy branch. This was the only acro to show tip death.
Now recovering and showing new growth colors.
And that green milli frag? It grew during the entire incident.
... and then 2 weeks ago rapidly lost half of it's base.
The way I look at it all of these corals were hurt and succumbed to different maladies due to their poor health, or like the first one above were able to recover on their own. The important thing to do is lock in stability again, frag if you have too, and don't over react to damage done weeks or months ago.
My KH is now stable at 8
PO4 < .06
As someone else has said to me, the trials of an acro keeper.
Good luck, hang in there!
