Abstract The biodegradable polymer poly-β-hydroxybutyrate (PHB) is a promising carbon source for biological mitigation of nitrogen pollution, a significant problem in aquaculture that physical and chemical methods have not provided a comprehensive solution. Here we investigated the impact of PHB on the zero-water-change largemouth bass culture by 30- and 40-day experiments. PHB loaded into the filter circulation pump at 4 g L− 1, optimum value determined by the first experiment, significantly reduced the levels of nitrate by 99.65%, nitrite by 95.96%, and total nitrogen by 85.22% compared to the control without PHB. PHB also significantly increased denitrifying bacteria (e.g., Proteobacteria and Fusobacteria) and expression of denitrification genes (e.g., nirK and nirS) in the microbial community, improving growth and health parameters of largemouth bass. While the impact may vary in other culture systems, PHB thus demonstrated its remarkable utility in aquaculture, highlighting ecological assessment and application to larger aquaculture operations as future considerations.