The production efficiency of rotifers, representing a crucial feed for fish larvae, is a key factor determining fish production in aquaculture. Poly(butylene succinate-co-adipate) (PBSA) is a biodegradable polymer that enhances rotifer growth, but its effect on bacterial composition remains unknown. This study was aimed to investigate changes in the structure and function of actively growing bacterial communities in rotifer culture water after PBSA treatment. PBSA induced rapid changes in water quality, including elevated organic carbon levels, decreased pH, and increased in bacterial abundance and respiration, suggesting the elution of oligomers and monomers. Consequently, these conditions led to the increased prevalence of certain actively growing bacterial families, notably Alteromonadaceae and Hyphomonadaceae, compared with that in water without PBSA, which persisted throughout the experiment. Moreover, a decrease in ammonium concentrations was observed after PBSA treatment. As functions predicted using PICRUSt2 indicated that bacteria influenced by PBSA were unlikely to directly contribute to nitrogen conversion processes such as nitrification, denitrification, and nitrogen fixation, this observation was attributed to enhanced bacterial assimilation. These findings help elucidate how PBSA enhances rotifer production, highlighting the importance of considering the functional roles of the potential microbial consortium created by PBSA to optimize its use in seed production.