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In the summer of 2018, this study was performed on 50 Koi fish transferred to Razef Research Complex. In this study, after observing macroscopic symptoms in fish, including lethargy and immobility, imbalance, darkening of the skin, skin wounds, petechiae in the abdomen, opacity of the eye and anophthalmia, anorexia, and sitting on the bottom of the aquarium, sampling was performed to diagnose the pathogens and examine the pathological changes. According to microbial tests on skin samples and internal organs, Aeromonas hydrophila was reported, and septicemia in 48% of the studied fish. In necropsy specimens, the organs were pale, the liver was enlarged, and petechiae were observed on the surface of the liver. The microbial contamination also led to considerable tissue damages, including the destruction of gill filaments, nuclear pycnosis in the liver, and the infiltration of inflammatory cells into the heads of all fish. Considering the significant losses due to the pathogenicity of Aeromonas hydrophila in different fish species and the importance of Koi fish, observing quarantine and hygienic principles to prevent sepsis was very important. Early diagnosis and use of appropriate treatment protocols for this disease are recommended. [ABSTRACT FROM AUTHOR]