I think my pleco has tail rot ; it looks jaggedy and definitely rotting away... I just did 25% water 2 days ago (and 5 days before that) so I don't think it's my water.
When I get this sort of problem, which normaly happens tbecause of damage from tank furnature or a dit of fin nipping. Rather than treat the whole tank I will catch the dammaged fish and treat the infected area with Methylene Blue, using a small artist type paint brush.