Hi Sophie,
Treating the pleco may not be necessary.....It may be dietary... do you have any driftwood in there for it? do you suppliment its feeding?
Sometimes plecos can eat too much of the other fishes food and if the food is old and rotting...get an internal infection.
depending on the above it could also be parasites in which case...it will probably be flagellates and Metronidazole (jehmco.com) would be a good start....it may also help if its bacterial.
hth,
al