Yes, it is just the fin.
My nitrates have been higher than I like since I added the new fish (20 -40). I do 25% PWC
every two weeks, cal and mag was low for awhile but it's back up, all the other numbers are great. And I check for EVERYTHING twice a week (my husband thinks I'm over protective of my fish). So I think I'm ok there unless it's the nitrates. I wouldn't think it could be another fish as much as he stays in his hole, or could it?
What is fin and tail rot?