Aquaculture, a rapidly growing global food-production industry, faces persistent challenges including diseases triggered by high stocking density, overfeeding, and substandard water quality, often necessitating the use of antibiotics and posing risks of immune suppression in fish. Probiotics show a viable alternative for disease mitigation and improved aquaculture practices. Major Objective: Thus, the present study evaluated the impact of probiotic (Lactobacillus rhamnosus) on the growth dynamics, proximate analysis, and enzymatic activity of grass carp (Ctenopharyngodon idella). Method:120-grass carp were subjected to two different groups (control and experimental) with triplicate having 20 fish in each. The experimental fish, receiving L. rhamnosus at a concentration of 3×10⁸ CFU/g, exhibited significant enhancements in weight gain rate of 137%, specific growth rate of 0.91%, survival rate of 77%, and final body weight increased by 50.4 g compared to the control group. The proximate composition demonstrated a significant increase in crude protein (46.32±0.96%) in the experimental group, while moisture content (64.43±0.72%), crude fat (34.30±0.40%) and total ash (9.80±0.80%) were reduced. However, dry matter remained consistent. Furthermore, a significant increase in digestive enzyme activity (amylase, protease, cellulase) was observed. Crucially, key water quality indicators such as dissolved oxygen, pH, and temperature remained unaltered between the experimental and control groups. Conclusion: These findings highlight the potential of L. rhamnosus to positively influence growth and well-being in aquaculture, emphasizing it as an antibiotic option for disease reduction and sustainable aquaculture.

Keywords: Probiotic, Feed supplementation, Growth performance, Enzyme activity, Lactobacillus rhamnosus and Ctenopharyngodon idella.