Which fin are we talking about?
Any idea what caused the damage?
It's the clear side fin in the photo with the white-ish blob mark on it.
When i first spotted it, the fish was doing a really weird wriggle, bending fully one side to the other very quickly - like it had been hooked and was fighting, almost. It was so odd, i stopped to watch, thinking it may be a display behavior. Only did it the few times i saw then behaved normally. Then i noticed a streamer of fine white goo/thread trailing from the side fin. It was tattered as well. It apparently got rid of the thread bit but the white mark remained, which is when i took the photo. I don't know what could have done it for sure but i have two suspicions:
1) it brushed against one of the many aiptasia in my tank and it got stung. I have no idea what the affect is on fish but it would explain the damage and goo stuff.
2) a suspicion that has been growing for a few days -
Newly cleaned out nerite snail shells + frags knocked over + corals removed from their tiles + loud clicks =
Mantis.
I'm wondering if suspected guest had a crack at the Bangaii.
On the plus side, after adding melafix and giving my flatworm infested frags a melafix bath, the fin is completely healed today. Fish is eating greedily and swimming fine still.
So.
Something to keep an eye on.