The African catfish (Clarias gariepinus x Heterobranchus longifilis) is one of the most important fish species in Hungarian aquaculture. The larvae and juveniles are reared at high feeding rates, which can reach up to 10% of the biomass. Intensive feeding results in large amounts of effluent water, directly proportional to the intensity of fish production. The aim of this experiment was to investigate the potential of Common duckweed (Lemna minor) as a biological filter and its nutrient removal capacity during intensive fry rearing of the hybrid African catfish. In a recent study, two experimental recirculation systems, identical in water volume and design, were set up, in one of which duckweed was grown under aquaponic conditions, where the concentrations of nitrogen and phosphorus forms were monitored and the production parameters of the fish were determined. The results obtained from the experiment showed that the use of duckweed as a biological filter improves the water quality of the recirculation system. A significant amount of plant biomass was produced, which mainly reduced the nitrate and orthophosphate concentrations in the system water. As a consequence, the survival of hybrid catfish was improved, which also affected biomass growth, but had no effect on other parameters.