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¹ Department of Nutrition and Clinical Nutrition, Faculty of Veterinary Medicine, Zagazig University, Al Sharqia Governorate 44519, Egypt. DOI : 10.4194/AQUAST2464 Viewed : 73 - Downloaded : 47 The use of natural feed additives presents an eco-friendly strategy to improve the performance and well-being of Nile Tilapia. A total of 180 juvenile Nile tilapia was randomly assigned to three treatment groups, each with three replicates (20 fish per replicate), and reared in glass aquaria. The groups were as follows: (1) control group, fed the basal diet; (2) probiotic group, fed the basal diet supplemented with 2 g/kg of Bacillus subtilis (2 × 10¹¹ CFU/kg); and (3) acidifier group, fed the basal diet supplemented with 5 g/kg of an organic acid blend. The organic acid and Bacillus subtilis supplementation boosted fingerlings` growth outcomes, protein retention and specific growth rate with superior nutrient digestibility unlike control group. In addition, fish fed the Bacillus subtilis fed groups displayed significant hematological benefits, such as lower serum cholesterol levels and increased high-density lipoprotein cholesterol (HDL-C) levels, distinguishing them from the other treatment groups. The administration of probiotics and acidifiers effectively strengthened the fish`s antioxidant system, reducing oxidative stress markers such as malondialdehyde (MDA) and enhancing enzymatic antioxidants including total antioxidant capacity (TAOC) besides superoxide dismutase (SOD) activity. Furthermore, the inclusion of probiotics and organic acidifiers enhanced the immune response, as reflected by increased serum IgM, lysozyme activity, phagocytic activity, and the upregulation of mucin-2 and interleukin-10 gene expression in the intestine. Concerning the intestinal microbiota, the Bacillus subtilis fortified group exhibited an increased lactobacilli DNA concentration unlike a decreased lactobacilli DNA concentration of Enterobacteriaceae. Overall, Bacillus and acidifier inclusion positively influenced performance, immune status, and gut bacterial composition, contributing to the economic viability of tilapia farming. Keywords : Nile tilapia, Organic acids blend, Growth, Expression of genes, Bacillus subtilis