Biofilms in aquaculture tanks pose significant challenges, hindering cleaning processes and contributing to antibiotic resistance. This study investigated the effects of four Pseudoalteromonas strains on flat oyster (Ostrea edulis) rearing, with a specific focus on biofilm control and microbial communities. After confirming the safety of these strains for O. edulis, we monitored biofilm development and bacterial communities during a 4-month sexual maturation period. Biofilm biovolume was quantified using confocal laser scanning microscopy (CLSM), and bacterial community composition was analyzed via 16S rRNA gene metabarcoding of both biofilm and seawater samples. Our results revealed differences in bacterial community structure between biofilms and seawater. Furthermore, the presence of specific Pseudoalteromonas strains significantly impacted the composition of bacterial communities within the tanks. β-diversity analyses demonstrated that each strain exerted a unique influence on the bacterial community structure. Some Pseudoalteromonas strains effectively reduced biofilm biovolume without negatively impacting bacterial richness or diversity. These observations suggest that certain Pseudoalteromonas strains can effectively control biofilm formation while maintaining a diverse and potentially beneficial microbial community in O. edulis rearing tanks. The use of these strains as additives in aquaculture systems could offer several advantages, including reduced cleaning time and costs and a potential decrease in biocide usage.