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The present experiment investigated the possibility of microbial protein production in 250l indoor tanks by manipulating C:N ratio in fish feed applied. Two different levels of protein feed (35% and 22% CP) resulting in C:N ratio of 8.4 and 11.6, respectively, were applied at 25g daily in each tank. Tanks were aerated and agitated continuously using a dome diffuser. The experiment was carried out for eight weeks. The biofloc development in terms of VSS and BOD₅ was better in the low protein fed tanks than in the high protein fed tanks. An estimated biofloc productivity ranged 3–5gCm⁻³day⁻¹. A 3-D image stained with DAPI indicates that the biofloc is comprised of hundreds of bacterial nuclei, size being ranged from 100 to 200μm. Biofloc quality was independent of the quality of feed applied and contained more than 50% crude protein, 2.5% crude lipid, 4% fibre, 7% ash and 22kJg⁻¹ energy on dry matter basis …