The Study was conducted on the effects of antibiotic on bacterial flora under laboratory condition. Oxytetracycline (OTC), the most widely used antibiotic, was fed to the Mrigal (average body weight 25g) at the rate of 2g/kg through fish diet and bacterial content was estimated for a period of 20days.Total 8 aquariums were used, where 3 aquariums for control and 5 aquariums for replication of the treatment. Physico-chemical parameters of aquarium water were determined where temperature,of culture aquariums were more or less similar.Before antibiotic treatment dissolve oxygen (DO), pH and total hardness was 7.80±0.10mg/l, 4.10±0.10,
and890.00±10.00ppm, respectively which reached a value 9.90±0.10mg/l, 5.90±0.10and 710.00±10.00ppm, respectively indicating the changes after 20 days. Prior to antibiotic treatment,
bacterial load was 2.90±0.06×103cfu/ml in aquarium water, 6.90±0.20×105cfu/g in fish gills, 4.70±0.10×107cfu/g in fish intestine,and 85.25±3.38×105in fish skin respectively which was
Significantly reduced to 1.25±0.03×103cfu/ml in water, 5.42±0.20×105cfu/g in gills, 3.33±0.05×107 cfu/g in intestine, and 11.24±0.01×105in fish skin respectively after 20 daystreatment period. Water and fish samples were also analyzed for bacteria were completely absent before and after antibiotic treatments.