The application of duckweed ( <scp> <i>Lemna minor</i> </scp> ) and freshwater mussels ( <scp> <i>Anodonta cygnea</i> </scp> ) as living biofilters integrating with a filtration system to maintain water quality in juvenile trout ( <scp> <i>Oncorhynch

Increasing nutrient concentrations in fish culture systems over time can reduce water quality. However, the nutrient increase can be remediated by pairing organisms at lower trophic levels with a mechanical filtration system to improve nutrient removal efficiency and water quality for fish culture. This research uses the RAS system to determine the performance of integrating living organisms as biofilters in rearing juvenile rainbow trout (Oncorhynchus mykiss) for 56 days. Duckweed (Lemna minor) was added to replicate tanks at three treatment levels: T1 (100 g wet weight and 20% area coverage), T2 (200 g wet weight and 40% area coverage), and T3 (300g wet weight and 60% area coverage). The duckweed in each treatment tank was supplemented with 20 freshwater mussels (Anodonta cygnea) with an average body weight of 56 ± 1.0 g. Physical and chemical water quality parameters were measured in fish tanks and all ponds in the RAS system. Fish from the rearing tanks were weighed every two weeks. Duckweed biomass was measured weekly; the mussels were weighed at the beginning and end of the study, and the mussels were measured at the beginning and end of the rearing period. The fish was partially harvested every two weeks to maintain constant fish biomass. Using duckweed (L. minor) with different biomass weights and areal coverage, coupled with the freshwater mussels (A. cygnea) as living biofilters, had a significant effect (P < 0.05) on water quality parameters. Ammonium (NH₄), nitrite (NO₂), and nitrate (NO₃) concentrations decreased throughout the study. During the study period, juvenile trout experienced growth with an SGR of 2.62-2.72%/gram with a survival rate of 100%. Partial harvesting during the rearing period positively impacted the average body weight of fish growth and duckweed biomass. The best duckweed growth performance was found in treatment T1 (cover area 20% with wet weight 100 g) with a productivity of 9.4 (g/m²/day). PRACTITIONER POINTS: Twenty percent duckweed coverage with freshwater mussels achieves optimal nutrient removal in RAS systems, improving water quality efficiently and growth better than other treatments. Combined biofilters (duckweed-mussel) and filtration units reduce operational costs while maintaining high fish survival rates in RAS systems. Integration of living biofilters provides sustainable water treatment without chemical additives, suitable for small-scale aquaculture operations.