To me it looks like a wound, but could be fin/scale rot. Hope it is NOT SFR, I treated the water with aquarium salt.
I am no expert, but back in 2015, I had a older male, had an eruption further up his "shoulder", looked like a pimple. Soon it popped and he was completely infected, scales AND fins deteriorated and fell away. He was lethargic and not eating. My mostly dark blue /purple boy turned pink and lost his blue, except on his face and the rotted edges of his fins. Some one, very knowledgeable, told me he would most likely not make it. SLowly, he just sat in his home and I could not out him down, but he did not seem to he unhappy, just very still. After 8-10 months from the beginning, he was closer than ever to moving over the bridge, and I brought home his "replacement", he was NOT having that.... His name was Rumble (Rumble Two, actually) and then enter Grumble. Ticked of my Rumble and he began flaring, chasing at that other guy, show him gill covers and, low and behold, Rumble began to eat, just to live and drive Grumble away! With in a month Rumble was getting some color back and even built a nest, after the addition of a 3rd boy, Gorg, to the mix, but all in separate containers.
Rumble survived another few months after that, but was a good 15-16 months after we thought he was a lost cause.... Rumble The Ruff and his nest (he was about 5.5 years old + when he passed)
Grumble,
Gorg, (AKA Gorgeous)
Rumble, Grumble and Gorg (Back to front)
Vick (AKA Vicious Fischious)
