I have never seen a parasite cause fin rot but anything is possible. For fin rot I use either Maracyn or Maracyn 2. Maracyn is for gram positive bacterial infections, whereas Maracyn 2 is for gram negative bacterial infections. Unfortunately, you won't know which kind of bacteria is causing the fin rot. Because of this, if the fin rot is severe I use both Maracyn and Maracyn 2 in combination. Be sure to use it for the proper amount of days, otherwise you may end up with antibiotic resistant bacteria that will be even harder to cure.