What killed my Coral Beauty?

[Taki]

current stock:

2 mature skunk clowns
1 small Ocelaris
1 Foxface
1 Lawnmower Blenny
1 Melanurus
1 Diamond Goby
I BTA
1 Hammer coral

Part of the fin is missing, and there’s a grey patch that doesn’t show well in the picture but it’s in the same area as the dorsal fin damage, located on both sides of the fish. It looks like something took a big bite or has been nibbling. Is it a sting from the anemone or hammer? Disease?

 

[Spieg]

Almost impossible to say for sure but whenever I hear about discolored patches and fin rot that affect only 1 animal, I think of bacterial infection. How did the symptoms initially present and progress?

 

[Taki]

Almost impossible to say for sure but whenever I hear about discolored patches and fin rot that affect only 1 animal, I think of bacterial infection. How did the symptoms initially present and progress?

Everything happened quite fast. I introduced the coral beauty along with a melanurus wrasse and a diamond goby 6 days ago. I noticed the symptoms only this morning. Other fish are doung fine.