Resilience of nitrification and fish health under repeated hydrogen peroxide addition in lab-scale RAS culturing juvenile Hexagrammos otakii

Abstract Hydrogen peroxide (H₂O₂) is widely used for disinfection in recirculating aquaculture systems (RAS), yet its effects on biofilter performance and fish health remain dosage-dependent. In this study, we evaluated the impacts of weekly H₂O₂ treatments (0, 5, 15, and 30 mg/L) over a 5-week period on water quality, nitrification, and the physiological condition of juvenile Hexagrammos otakii in lab-scale RAS. Low concentration H₂O₂ (5 mg/L) enhanced specific growth rate (SGR: 1.38 ± 0.30%/day) and improved feed conversion efficiency (FCR: 0.93), without significant disturbance to TAN, NO₂-N or NO₃-N levels. In contrast, higher concentrations (15 and 30 mg/L) caused transient but marked disruptions in nitrification, with TAN peaking at 2.5 mg/L and NO₂-N exceeding 3.0 mg/L. Histopathological analysis revealed dose-dependent tissue damage in the gills and liver, particularly under 30 mg/L H₂O₂, accompanied by reduced SGR (0.78 ± 0.23%/day) and elevated mortality. Interestingly, nitrifying bacteria showed signs of adaptive recovery after repeated exposures, as evidenced by gradually shortened recovery times in TAN and NO₂-N levels following each treatment. These results provide further insights into safe operational ranges for H₂O₂ use in RAS, suggesting that low-dose, intermittent application can offer disease control benefits without compromising system stability or fish welfare. The findings also highlight the potential resilience of nitrification processes under oxidative pressure, supporting the development of robust RAS management strategies.