This study aimed to investigate the optimize conditions for tannin extraction from mangosteen peel and to evaluate the property in ammonia removal in fishpond water. Ground mangosteen peel, at the concentrations of 10, 20, and 30 grams/L, were soaked in water for 0, 1, 5, 7, 14, 21, and 28 days. Tannin in each condition was analyzed. The second experiment was tannin extraction from the mangosteen peel by boiling. Finely ground and crushed mangosteen peel at the 10, 20 and 30 gram/l were boiled at different times for 0, 30, 60 and 120 minutes. Tannin from these conditions was determined. It was found that conditions of 30 gram/L and the 120 minutes boiling time could be used for extraction providing the highest tannin concentrations of 5,075 mg / L (P <0.05). The efficiency of ammonia removal by tannin extract was then investigated. The result revealed that mangosteen peel-tannin extract could mitigate toxic ammonia in fishpond water. The ammonia removal rate was 0.073 ppm total ammonia/1 ppm tannin. Lethal concentration (LC50) of tannin extraction from mangosteen peel was 296 ppm, and the safe tannin concentration was 29.6 ppm. However, high concentrations of tannins should let the pH of the water to drop rapidly that causing stress to fish. Therefore, applying of tannins in fishpond should be at a suitable amount to maintain ammonia appearing.