Rapid growth of marine fish farming often leads to adverse impacts on the environment, especially the release of dissolved nutrients which may cause eutrophicaton.As an ecological treatment solution, large seaweeds can be used to get rid of the excess nutrients while obtaining higher output.A setup consisting of six 3m3 tanks(one with neither fish nor seaweeds, others with or without seaweeds) was used to investigate ecological function of polyculture of fish and seaweeds.As the experiment period was spring, Gracilaria lemaneiformis grew well even in the low nutrient environment; and had average specific growth rate(SGR) of 4.05% in the four polyculture tanks, with the highest one (4.5%) in the #4 tank.In addition, except the # 3 tank,G.lemaneiformis had competitive advantage over phytoplankton after a period of time.At the same time, most of the dissolved nutrients(NO 3-N,NO 2-N,NH 4-N,PO 4-P)in the polyculture tanks were removed as the enviromental dissolved inorganic nutrients(DIN) were much lower than those of the tank without G.lemaneiformis, whose average content was 2.53 μmol/L.In order to achieve good water quality as well as high production, carrying capacity has to be considered according to existing system and conditions.In this experiment, 1∶1 co-culture of G.lemaneiformis and Sebastodes fuscescens yielded good results. Rapid growth of marine fish farming often leads to adverse impacts on the environment, especially the release of dissolved nutrients which may cause eutrophicaton. As a ecological treatment solution, large seaweeds can be used to get rid of the excess nutrients while obtaining higher output. A setup consisting of six 3m3 tanks (one with neither fish nor seaweeds, others with or without seaweeds) was used to investigate ecological function of polyculture of fish and seaweeds. As the experiment period was spring, Gracilaria lemaneiformis grew well even in the low nutrient environment and had the average specific growth rate (SGR) of 4.05% in the four polyculture tanks, with the highest one (4.5%) in the # 4 tank.In addition, except the # 3 tank, G. lemaneiformis had competitive advantage over phytoplankton after a period of time. At the same time, most of the dissolved nutrients (NO 3 -N, NO 2 -N, NH 4 -N, PO 4 -P) in the polyculture tanks were removed as the enviromental dissolved inorganic nutrien ts (DIN) were much lower than those of the tank without G. lemaneiformis, whose average content was 2.53 μmol / L. In order to achieve good water quality as well as high production, carrying capacity has to be considered according to existing system and conditions.In this experiment, 1:1 co-culture of G.lemaneiformis and Sebastodes fuscescens yielded good results.