A wide range of pathogens is associated with ornamental fish diseases. Among the bacteria, Aeromonas hydrophila causes disease under stress conditions or in concert with infection by other pathogens in ornamental fish. The present study was undertaken to the Aeromonas hydrophila pathogenesis of goldfish (Carrassius auratus) and Koi (Cyprinus carpio koi), during massive fish disease outbreaks in various infected ornamental fish hatcheries in Kanyakumari District, Tamilnadu, India. Also the virulence factors such as protease, proteolytic activity and haemolytic activity was studied the virulence strains compared with the non-virulent A. hydrophila strains. The virulent A. hydrophila was challenged with the fingerlings of ornamental fish and the virulence was discussed.