Koi, with their captivating beauty, cultural significance, and investment potential, have transcended national borders to become a truly international commodity. The global koi trade not only generates economic benefits but also promotes cultural exchange and appreciation, fostering a shared passion for these remarkable fish. However, the threat of a serious disease, Motile Aeromonas Septicemia (MAS ), caused by the bacteria Aeromonas hydrophila can be fatal to koi if not treated properly.
This study was carried out descriptively using samples from the Koi Fish Farming Centre in Blitar , East Java. Fish samples were collected weekly for 5 weeks. Water quality and tissueof gills and tail skin damage caused by Aeromonas hydrophila bacteria were carried out at the Bioscience Laboratory, Brawijaya University, Malang. This research was conducted from January to May 2024.
Scanning electron microscope observations of the tail of Aeromonas-infected koi fish provide a detailed picture of the cellular damage that occurs. Aeromonas infection in Koi fish causes various types of cellular damage, such as mineralisation, necrosis, hyperplasia and vacuoles (Table 1) . Histopathology of the gills shows changes in the structure of the gill tissue such as telangiectasis, secondary lamellar oedema, necrosis and hyperplasia.
The dynamics of water quality, such as DO, pH and salinity during the observation were quite varied. However, the water temperature was slightly higher than the Indonesian national standard for Koi farming. In addition, high levels of TOM and ammonia in the pond water are also suspected to trigger MAS infection in Koi fish.