This study attempted to assess the potential use of common duckweed Lemna minor asnatural biofilter in brackish closed recirculating systems of 4 g/L salinity and to evaluate the effect ofsalinities in the ranges of 1-7 g L-1 on the duckweed’s ammonium NH4 +1uptake. Furthermore thepossibility of nitrification as a second mechanism of nitrogen removal in closed recirculating systems wasinvestigated. Three closed recirculating systems each having 20 mixed Oreochromis niloticus were used.Duckweed L. minor and fresh water were added in system 1, fresh water only in system 2, and duckweedwith brackish water in system 3. Ammonium NH4 +1and nitrate NO3-1 were tested periodically in eachsystem during a 1.5 month period. Another three experiments were run in parallel using aquariumsincubated with 60.00 g fresh weight L. minor at salinities in the ranges of 1-7 g L-1 for two weeks.Aquariums were exposed directly to sunlight in experiment 1 and 3, and were placed in the dark inexperiment 2. The nitrogen source in experiment 1 and 2 was 100 mg L-1 of NH4Cl, and 280 mg L-1 ofNH4Cl in experiment 3. NH4 +1and NO3-1 levels were monitored as above. L. minor managed to reduce theaverage NH4 +1levels to 0.5 mg L-1, 0.43 mg L-1below the standard recommended levels in both brackishand fresh water systems, respectively. Ammonium NH4 +1 uptake was coupled with nitrate NO3-1 uptakeunder favorable conditions of sunlight. Salinity in the ranges from 1-7 g L-1 enhanced ammonium NH4 +1uptake (r = 0.8819, p = 0.023) without affecting nitrate NO3-1 uptake or any observed mortality of theduckweed. Nitrification was a second mechanism of nitrogen removal in a closed recirculating system,but it was affected by salinity and light. Average ammonium NH4 +1of 51.8 mg L-1 was toxic to L. minorand death was observed within a week in experiment 3. The duckweed L. minor was an efficient naturalbiofilter in both brackish and fresh water closed recirculating systems.