Never treated for brook. She is almost 16 years old though.
I am uploading a video to make it a bit easier. It will need a few moments to process but it was shot in 4K.
I had this same situation about a week ago with a spot on her tail which went away. I’m currently adding selcon to one of her two meals a day, rotating through frozen, flake and pellet.
I didn’t notice this at lunch today (like all of us I look everything over and since she’s one of two fish, I notice if somethings different) and I was doing work in the tank this evening after work and she attacked me, there is a likelihood that she ran into one of the rocks.
What about a freshwater dip?