Trichodina are circular, flat ciliates with circular rows of denticles (internal blades) and are commonly found living on the skin and gills of many fish.  They usually do no harm but when poor water quality or other stressors are present, they may cause disease.  This parasite is more common on pond raised fish. Treatment with salt, malachite green, formalin and potassium permanganate are usually effective, however some strains have shown resistance.