Abstract:The present study evaluated the probiotic potential of Bacillus velezensis STPB10, isolated from the gut of Schizothorax richardsonii, with emphasis on water quality improvement in freshwater aquaculture. Common carp (Cyprinus carpio) fingerlings were reared in tanks supplemented with STPB10 at 3.0 × 10⁸ and 3.0 × 10⁹ CFU mL⁻¹, alongside an un-supplemented control. Probiotic addition, particularly at 3.0 × 10⁹ CFU mL⁻¹, significantly reduced nitrite (125 → 30 ppm), nitrate (≥45 → 10 ppm), phosphate (2 → 0.2 ppm), and CaCO₃ (≥45 → 30 ppm) compared to the control. The improved water stability was directly associated with fish survival, with the highest resilience observed in the 10⁹ CFU mL⁻¹ group, partial survival in the 10⁸ group, and complete mortality in the control. These findings highlight B. velezensis STPB10 as a promising water-enhancing probiotic for sustainable freshwater aquaculture. STPB10 also demonstrated robust adaptability with survival rates of 31–75% across acidic pH (2–4), >99% survivability in 0.3% bile salt, and tolerance to 15–35 °C. It produced strong biofilms, adhered efficiently to intestinal mucus (63.67%), and significantly reduced the adhesion of major pathogens (Aeromonas hydrophila, A. veronii, A. salmonicida, and Vibrio anguillarum). The strain was γ-hemolytic, antibiotic-susceptible, and non-pathogenic to common carp, confirming its safety.
Overall, supplementation of B. velezensis STPB10, particularly at 10⁹ CFU mL⁻¹, markedly improved rearing water quality and fish survival, establishing it as a promising probiotic candidate for sustainable freshwater aquaculture.