Watermeal (Wollfia arrhiza) is a small aquatic plant which we have used to treat water from black tiger shrimp ponds. The relationship between watermeal biomass and treatment length, the changes in water quality parameters, and N-balance were evaluated for the treatment of black tiger shrimp farm effluent in low-salinity areas. A biomass of 12 g of watermeal per litre of shrimp farm effluent and a treatment period of 30 days were found to provide the best conditions for the growth of watermeal, and the quality of the treated effluent in terms of biological oxygen demands, suspended solids, total phosphorus, nitrate, total ammonia nitrogen, and total Kjeldahl nitrogen. The pH and salinity were similar for each level of biomass. The watermeal biomasses of 4-12 g per litre of effluent were suitable for watermeal survival over time. Since watermeal can fix nitrogen from the atmosphere, it can grow very well in effluent containing a low level of nitrogen, maintaining the N-balance.