In the initial quantification, pacu presented an average of 173.9±80.4 Trichodina fish
, while tilapia numbers were higher than 300Trichodina fish . No trichodinids were found on the gills. The four Trichodina species were T. magna, T. compacta and T. centrostrigeataon tilapia and T. heterodentata on pacu. In tilapia, the reduction was 87.9%, from an average of 140.0±1.4 parasite fish in control to 17.0±24.5 parasite fish in fish exposed to 50.0 mg TFB L −1 for one hour. In pacu, we observed a reduction of 96.1%, with an average of 176.3±80.9 parasite fish in control and 6.5±10.64 parasite fish after treatment with 80.0 mg L−1 for one hour (Table 1). Tilapia survival during the efficacy experiments ranged from 60.0% to 90.0% in the groups treated with TFB, whereas the fish from non-treated groups had a survival rate of only 13%. For pacu, the survival rate ranged from 86.0% to 100.0%, and no mortality was observed in the non-treated group