Mangosteen peel (Garcinia mangostana) contains antibacterial compounds such as tannins, alkaloids, flavonoids, and saponins that have the potential to treat bacteria-infected fish. This study was conducted from December 2024 to April 2025 at the Fish Parasite and Disease Laboratory, Faculty of Fisheries and Marine Sciences, University of Riau. The objective was to observe the gill and kidney ti…