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Microalgae plays a diverse role in aquaculture and chlorella sp. is a predominant microalga involves in the aquatic food chain. The present study focus on the preparation of microalgal fish feed, study of proximate composition and control of bacterial infection in fish. For the present study tilapia was selected which is most consumed and easily infected by pathogen. In the current study phytochemical analysis reveals that the petroleum ether and ethanol was the best solvent for the extraction of bioactive constituents. The antibacterial activity of ethanol extract of micro algae showed highest effect against staphylococcus aureus, klebsiella sp, shigella sp and E coli. The highest antibacterial activity was noted against staphylococcus with the zone of inhibition of 1.2 ± 0.1mm. The antioxidant activity showed that the ethanol extract of C. vulgaris possess high scavenging activity of 40.5%. The extracts showed good reducing power ability in a dose dependent manner. A medicated algal feed was prepared and administered to the fish to study its pharmacological activity. It was noted that the amount of proximate composition was drastically increased after fed with medicated feed. To study the efficacy of feed to control staphylococcal infection, the fish was injected with staphylococcus. Tilapia fed with medicated feed showed higher survival than the control group and it could effectively control the fish bacterial pathogen staphylococcus. The present study revealed that the microalgae fish feed is an excellent source of nutritional supplements for the fishes.