This study evaluated the protective effects of pomegranate peel, garlic, and a polyherbal mixture (PHM) against Aeromonas veronii-associated gill disease in common carp (Cyprinus carpio). A total of 234 healthy fish were allocated into 13 groups, including one uninfected control group and 12 experimentally infected groups. Infected fish received diets supplemented with oxytetracycline, garlic (2.5%, 5%, and 7.5%), pomegranate peel (2.5%, 5%, and 7.5%), or PHM at equivalent concentrations for 14 days. Untreated infected fish and garlic-treated groups exhibited 100% mortality within 14 days post-infection. In contrast, PHM supplementation significantly improved survival, achieving up to 94% survival (P < 0.05). PHM-treated fish also demonstrated improved hematological parameters, enhanced immune-associated responses, and reduced alterations in liver enzyme activities compared with infected controls. Histopathological examination revealed that PHM preserved gill architecture and reduced epithelial degeneration, necrosis, and hyperplasia associated with A. veronii infection. These findings demonstrate that dietary PHM supplementation enhances resistance to A. veronii infection and improves several health-related parameters in experimentally challenged common carp. The observed protective effects may be associated with the bioactive phytochemical constituents identified by GC–MS analysis; however, the precise mechanisms of action remain to be elucidated. Furthermore, additional studies are required to evaluate long-term safety under non-challenged conditions and to further characterize the biological activities of PHM.