I agree with all the good advice about rearranging tank mates and etcetera. However, I do not agree with the medications recommended. I place more trust in the use of known effective medications. Doing nothing works fine if there is no secondary infection beyond purely mechanical damage but damaged fins are open invitations for infections. You can take the chance and not treat but more fish that receive appropriate treatment will pull through than those not receiving any treatment.
I would treat the Pleco in isolation with acriflavine and gradually increase adding rock salt from 1 tsp/gal to 1 tbs/gal. Begin at the lower dose then add the rest after 24 hours. Acriflavine has good bactericidal properties so it can often stop the progression of the rotting and salt seems to help begin the healing of damaged fin tissues. It is hard to provide a recommended dose but since it is a fluorescing dye use enough to make the water yellow enough to be difficult to see well through the water. It is sold in so many different concentrations there is no exact dose I can provide.
It also has anti-fungal properties and can prevent fungus infections starting growing along the damaged edges of the fins. It is pretty safe so the exact dose is not very critical although too low a dose will not help much. Maybe this will help provide some guide lines.
http://www.fishyfarmacy.com/products.html#A
Healing should begin to become apparent within a couple days by the time the salt is at 1 tbs/gal.
Ideally, I would recommend Chloramphenicol HCL at 250 mg/5 gal. Unfortunately it is difficult to obtain although it may be possible to get some through a Vet by prescription. I can almost guarantee that will work if you can get some.
Using salt would be optional if you can obtain this antibiotic. The salt never hurts for treating fish with bad scrapes and/or damaged fins.
Oxolinic Acid is another effective drug and is much easier to obtain than chloramphenicol. It is good but not as much as chloramphenicol. It has a similar molecular structure as chloramphenicol and is widely used by the Koi Carp breeders and hobbyists for treating bacterial fin infections.
http://www.fishyfarmacy.com/products3.html#O
The antibiotics and antibacterial drugs available over the counter have become nearly useless after decades of over use. I do not consider Melafix or Pimafix as effective medications. In nearly every case where they have been used and the fish recovered they probably would have recovered anyway. This has led to some thinking the medications were responsible. They do no harm but they also have not been proven to be effective. I know of no controlled tests that prove their efficacy.
Avid Trout fly fisherman. ·´¯`·...¸><)))º>
