Application of dietary humic acid and fulvic acid to enhance growth performance, gut health, and microbiome of Nile Tilapia raised in biofloc systems

Objective Humic substances are potential feed additives for use in the aquaculture industry, as they may enhance growth performance and health status. The present study aimed to evaluate the effects of dietary supplementation with fulvic acid and humic acid on growth performance, gut health, and microbial communities in Nile Tilapia Oreochromis niloticus raised in recirculating biofloc systems. Methods A feeding trial was conducted for 60 d, using 540 juvenile tilapia that were distributed across 18 tanks (30 fish/tank). Three experimental diets, isonitrogenous (40% crude protein) and isolipidic (10%), were formulated, including the control, a diet with 1.5% fulvic acid, and a diet with 0.5% humic acid. Each dietary treatment was assigned randomly to six tanks. Results A clear difference among dietary treatments was observed after day 45, with humic-acid-fed fish showing better growth performance than both the fulvic acid and control groups in final weight, weight gain, and percent weight gain. Better feed utilization, as indicated by feed conversion and protein efficiency ratios, was observed in the fulvic- and humic-acid-supplemented groups compared with the control. The whole-body nutrient composition was similar among treatments except for crude protein. Compared with the control, there were no significant differences among treatments for blood chemistry parameters and serum lysozyme activity. Moreover, the qualitative evaluation of distal intestinal histology did not show any difference among treatments. Except for the observed richness, there was significant alpha diversity variation in the fish gut, and no treatment effect variation was observed in biofloc water. No differences were found in beta diversity in fecal and gut samples. While the phyla Bacillota, Planctomycetota, and Verrucomicrobiota were more abundant in the humic-acid-treated group than in the control group, no notable difference was observed in water samples across treatments. Conclusions Overall, the treatment group with 0.5% humic acid supplementation improved growth performance from 45 d of the growth trial through the final sampling (day 60), with no notable differences in lysozyme activity or blood parameters. This study provides comparative data on dietary supplementation with humic acid and fulvic acid in recirculating biofloc systems for Nile Tilapia.