Nanoparticles are of great interest because they are extremely small in size and large in surface-tovolume ratio, which lead to both chemical and physical enhancements in their properties compared to their bulk composition. Tremendous growth in nanotechnology has opened up novel, fundamental, and applied frontiers in material science and engineering, nanobiotechnology, surfaceenhanced Raman spectroscopy, and applied microbiology. In this study, the cell free extract of four micro algae were used for the biosynthesis of Ag-NPs. The size, shape and antimicrobial activity were also determined. The effectiveness of the four silver nanoparticles (Ag-NPs) produced by microalgae and biosynthesized and coated on activated carbon (AC) granules was evaluated in fish culture tanks. The cell-free extract derived from the various species of microalgae influences the size, shape, and antibacterial activity of the silver nanoparticle. Scanning electron microscopy was used to characterise the Ag-NPs/AC that was produced (SEM). The synthesised Ag-NPs/AC had effective antibacterial action against aquatic diseases, and its efficacy may be increased by adding a cell-free extract from the chosen algal species. A silver nanoparticle-coated activated carbon filterfabricated in this study was found to be cost-effective and simple-to-use as an antibacterial water filter in ornamental fish culture tanks.