One of the species of freshwater aquaculture that is much loved by local and foreign people is the koi fish (Cyprinus carpio). However, koi fish farming is often attacked by a bacterial disease caused by the Aeromonas hyrophila bacterium, which becomes a serious problem because this bacterium can cause death in koi fish. Papaya leaves were chosen as an alternative medicine in the treatment of koi fish (Cyprinus carpio) infected with bacteria, given that papaya leaves contain many compounds that are antibacterial and do not have side effects when used. In this study, the papaya leaves used were young papaya leaves which were extracted using the meseration and distillation methods. The purpose of this study was to determine the effectiveness in treating koi fish infected with Aeromonas hydrophila bacteria and what concentration of papaya leaf extract is effective as an antibacterial in treating koi fish seeds infected with Aeromonas hydrophila bacteria. Papaya leaf extract was administered at different concentrations, namely 0 ml (control), 5 ml, 10 ml, 15 ml for 30 days before and after being infected with Aeromonas hydrophila bacteria. The result is that papaya leaf extract has antibacterial effectiveness in inhibiting the growth of Aeromonas hydrophila bacteria at a concentration of 15 ml with a survival rate of 95%. The higher the dose of papaya leaf extract, the higher the effectiveness against Aeromonas hydrophila bacteria.