Efficacy of Gracilaria verrucosa extract baths against Trichodina sp. in comet goldfish Carassius auratus

Trichodina sp. is a major ectoparasite affecting Carassius auratus, reducing its aesthetic and market value. This study evaluated the efficacy of Gracilaria verrucosa aqueous extract against Trichodina sp., its impact on fish survival, and the effective dosage. The experiment was conducted using a Completely Randomized Design (CRD) with four treatment groups: K (methylene blue, 0.6 mL/L), P1 (G. verrucosa extract, 10 mL/L), P2 (G. verrucosa extract, 15 mL/L), and P3 (G. verrucosa extract, 20 mL/L), each with three replications. Observations included clinical signs, behavior, parasite prevalence and intensity, time to parasite death, fish survival, and water quality. Infected fish showed excess mucus, scale loss, pale gills, and behavioral changes. Treatment P3 significantly reduced parasite intensity to 2.93 individuals/fish, caused parasite death within approximately 8.5 minutes, and resulted in 100% survival. Parasite lysis was observed in treatments K, P2, and P3. Water quality remained within suitable ranges, with temperature at 26.5–27.9°C, pH 7–8.1, and dissolved oxygen 4.8–5.8 mg/L. These findings suggest that G. verrucosa aqueous extract at 20 mL/L is effective against Trichodina sp. and safe for comet fish.