Gorgonian: aspergillosis or rapid tissue necrosis?

[dendro982]

Can you help with identification and treatment: new Swiftia (non-photosynthetic Caribbean gorgonian) rapidly loses tissue. I cut off seemingly undamaged branches in the afternoon, next morning tissue receded from 1/4" to 1" (6mm to 2.5cm).
Photographs allowed to see with magnification - it seems to me, that almost all parts are already affected (mottled).
Looks like tissue becomes mottled, losing color (become yellowish), then border becomes dark red (or not, in other places), then greenish, then white, then melts, leaving dead skeleton:

The search makes think about aspergillosis of the Caribbean corals, speed of progress - about rapid tissue necrosis. Not much information about treatment.

Can somebody help with ID and what to do: fungal or antibacterial treatment?

P.S. The tank is old and stable, other inhabitants (corals and invertebrates) are feeling well so far. I know about benefits of quarantine, but wanted to provide the stable environment to this coral

 

[dendro982]

Update: after second surgery - removing the affected parts and adjacent 3/8"(1cm) of the branch, process looks like stopped, or significantly slowered.
Half of the corals (frags, actually) had 30" freswater dip (don't like bleaching, that appeared after that), the other half - applied permanganate to the cut and the skeleton below. Had nothing else at home, and on national holiday all stores were closed. Now bought Primafix and Melafix, but still didn't use it, unsure about clams and feather dusters in the same tank.