The recirculating aquaculture system (RAS) is an ever-developing technology for producing fish with a low environmental impact. However, off-flavors can be a major problem in RAS fish production. Off-flavor compounds are of microbial origin and are accumulated in fish flesh. They typically cause a musty and earthy taste and odor, which consumers find unacceptable. Here we hypothesized that oxidizing compounds such as ozone (O3), hydrogen peroxide (H2O2), and their combinations, referred to as advanced oxidation processes (AOP)s, can remove or decrease these compounds in water and prevent their accumulation in fish. In this study, four different oxidative treatments (O3 low (0.4 mg O3 L−1), O3 high (0.8 mg O3 L−1), H2O2 (0.15 µl L−1), AOP (0.4 mg O3 L−1 & H2O2 0.10 µl L−1), and controls were applied to 10 experimental RASs for four months. The results showed that the treatments can reduce dissolved organic carbon (DOC) and the off-flavor compounds (geosmin, GSM and 2-methyl isoborneol, MIB) in circulating water, but they were not able to prevent the accumulated off-flavors in fish flesh below the sensory threshold. There was no significant difference in off-flavor removal between the treatments, which indicates that O3 treatment was ineffective in these conditions. However, H2O2 could still reduce the off-flavor concentrations in water.