In 2016, Barramundi, Lates calcaifer juveniles imported from Thailand to the aquaculture station at Sultan Qaboos University exhibited remarkable external clinical signs of losing scales, severe muscle necrosis, and ascites. Fin rot was also observed in the diseased fish. The mortality reached more than 90% and it was assumed that some bacterial species were associated with the diseased fish. The objective of the present study was to investigate the infection route of disease, revealing the causative agent and finding the most effective antibiotic treatment. The suspected pathogen vehicle was mosquito larvae in the outlet of the culture tank as bacterium was isolated inside the mosquito larvae. Primary, traditional phenotypic tests and the vitek test confirmed that the bacteria were pathogenic Aeromonas sobria and Lactococcus garvieae. Eight of the most commonly used antibiotics in the aquaculture industry was used for antibiotic susceptibility test. It showed that that Gentamycin was the most effective antibiotic while the most effective environmentally friendly source was effort of henna, Lawsonia inermis, at a concentration of 10%.