I wish I could help as I have had a very similar experience recently with the exact same looking symptoms. Green Chromis really aren't known for beating each other up in my experience and I had 5 green chromis - 3 anthias and a six line wrass all succumb to something that looks the same as what you have going on there.
All my water params were good as well:
0 Ammonia
0 Nitrite
Nitrates at 5 pmm.
PH 8.13
Temp 77
Salinity 1.025
All my fish have died except for two of the Chromis and 1 Anthias. I thought from the symptoms on one of the fish that it looked like they were infected with Brook so treated as described here elsewhere (formulin), but to no avail.
It disheartening to lose the fish and diagnosis (at least all of the symptoms I can find) suggested either Brook or Velvet, but the treatsments for these two diseases are different - I thought Brook, but maybe I erred and Velvet is the culprit? I don't have any pictures to post, but my fish looked exactly like yours prior to and just after death.
The last three fish are holding on, but only the two remaining chromis are eating and I suspect the death toll isn't over yet. Fish are in QT with treatment, but have not improved in past 24 hours.
If anyone can give us a lead here it would be greatly appreciated - I didn't get into this hobby to kill fish, but not feeling the greatest about this experience currently.
Incidentally there is nothing in the QT that could have done this and there are no signs of aggression in the QT, just sick fish.
This killer was very fast though - all fish except last three have died in the last four days -which is why I suspect Brook or Velvet.
Any help?
Thanks!
