Abstract: The bacterial population of water from the Marine Research Aquarium was examined qualitatively and quantitatively. Lower concentrations (1.3x102) were found in the initial phase of the aquarium tank and increased moderately day by day. A total of 1109 strains were isolated from 30 water samples. The isolates comprised of 16 species 10 genera; 86.65% of these isolates were gram-negative rods. The Vibrio isolates comprised 48.9% of the total population. The generic compositions of the bacterial isolates were Flavobacterium, Pseudomonas, Salmonella, Bacillus, Staphylococcus, Alcaligenes, Aeromonas, Enterobacter and Cytoplaga. The results show that the survival rate of the fish species Pomacanthus imperator, P. semicirculatus, Chaetodon collare, C. vagabundus, C. plebius are poor probably due to abundance of the Vibrio population. The higher concentration of other genera may also increases the mortality rate among these ornamental fishes. Majority of the bacterial strains isolated are also considered as human pathogens.