This experiment aimed to determine the efficacy of fulvic acid (FLA) on growth performance, innate immune system, antioxidant parameters, and expression of immune and antioxidant-related genes in zebrafish (Danio rerio). To this end, 12 tanks (3 per group), each containing 50 zebrafish (with an average weight of 85.7 ± 10.05 mg) in 72 L, were assigned to diets containing FLA at four levels: 0 (control), 0.25 (FLA1), 0.5 (FLA2), and 1 (FLA3) g/kg diet. Following an eight-week culture period, no significant differences in growth performance were observed among the treatment groups (P >
0.05). However, lysozyme activity, total immunoglobulin (Ig), and total protein concentrations in whole-body extracts were significantly enhanced in the 0.5-1 g FLA/kg diet groups compared to the other treatments (P <
0.05). No significant differences were observed among the groups in catalase (CAT), glutathione peroxidase (GPx), or superoxide dismutase (SOD) activities (P >
0.05). The supplementation of FLA significantly upregulated the gene expression of interferon-α (IFN-α) and tumor necrosis factor-alpha (TNF-α), with the highest expression observed in the 0.5 g FLA/kg diet group (P <
0.05). Additionally, interleukin 1 (IL-1) expression was markedly elevated in this group in comparison to the other treatments (P <
0.05). While there was a significant increase in GPx gene expression with dietary FLA (P <
0.05), no notable differences were observed among FLA treatments (P >
0.05). CAT gene expression remained consistent across all groups (P >
0.05). In contrast, SOD gene expression significantly increased in response to all FLA-supplemented diets, with the highest level observed in the 0.5 g FLA/kg group (P <
0.05). These findings suggest that FLA may serve as an effective dietary supplement to enhance the immune response and antioxidant capacity in zebrafish.