Hiya Diana, I have had a look at the info now, but I am not sure if it is that or not. The black mark looks like a really small ulcer, although I cant see if it is indented at all or smooth with the skin.
The red lines are still there but I dont know if that is fin rot or not. All other fish seem ok, just the molly that is a bit lethargic. Should I still treat or leave it a while?