[geekdafied]

That doesnt sound like fin rot, that sounds like a evil damsel. They will destroy clowns fins (among other fish) when they are bullying them. Very common. I had a domino damsel that was about 3"s long do that to a 3" maroon clown, and maroons are just as evil. They would fight all of the time and I finally moved both to a 29g tank where they could only bully themselves and no other fish. Th only other fish in there was my orange toadfish which none of them messed with. The toadfish ate both of them eventually, lol.