The half-smooth tongue-sole (Cynoglossus semilaevis Gunthe) is commonly reared in a recirculating aquaculture system. We evaluated the effect of the air-to-liquid ratio on the purification efficiency, DO, and pH in the biological aerated filters of a recirculating system. The test system was maintained at (19 ±1)the removal rate of TAN increased from 35.0% to 52.0%, and the removal rate of NO₂-N increased from 8.2% to 44.6%. Thus, the air-to-liquid ratio had a significant impact on nitrification, whereas the influence of chemical oxygen demand (COD) removal rate was not significant. The average removal rate was 10.14%. The pH values increased from 7.97 to 8.08 but decreased between the biofilter inlet and the outlet. The optimal air-to-liquid ratio was 1.25:1. This resulted in DO levels at the inlet of the first bio-filter tank that were close to saturation. Between the first bio-filter tank and the final bio-filter tank, DO only decreased by 10%. The recirculation aquaculture system maintained the water pH value between 7.9 and 8.1. Our results provide a reference for optimization of bio-film process culture conditions.