Effect of C/N ratio on growth performance, water quality and microbial community in water-reusing biofloc system for Crucian carp, <i>Carassius auratus</i>

Biofloc technology (BFT) is an innovative ecological aquaculture approach designed to reduce nitrogen concentrations, provide an in-situ food source, and remove pollutants from culture systems. This is achieved by manipulating the carbon-to-nitrogen (C/N) ratio by adding carbon sources to the aquaculture water. In this study, three C/N ratios were evaluated: 8:1 (group A), 18:1 (group B), and 22:1 (group C). The results showed that group C performed best in terms of end mean weight (78.33 ± 1.79 g) and specific growth rate (1.08 ± 0.08%/d), which were not significantly different from group A but significantly better than group B (P&lt; 0.05). Water quality analysis showed that nitrite nitrogen concentrations were significantly lower (P&lt; 0.05) in groups B and C than in group A. The diversity indices of groups B and C were significantly higher than those of group A (P&lt;0.05). At the phylum level, Proteobacteria occupied a dominant position in all groups, while Bacteroides accounted for 29.9% in group C. At the genus level, Flavobacterium and Sphaerotilus were the dominant bacteria in group C. The research shows that adjusting the C/N in the water to 22:1 can optimize the biofloc flora structure and promote the growth of crucian carp.