A drastic increase of human population and urbanization with demands of lots of waters have been produced a countless industrial and domestic wastewater. They finally evoked the eutrophication and its phenomenon like cyanobacterial and diatomal bloom in lakes and streams in world-wide. Dense bloom and organic material in water column also serially make a bad scene and order, and then make lots of water problems. Many countries have been tried to diminish the algal bloom and clean water quality of eutrophicated waters. However, numerous mechanical (physical and chemical) treatment methods are non-economic and rather caused a new water pollution, called a secondary pollution. This study is aimed to control the organic material or dense algal scum with pan-ecological technology, to select the relevant biological control agent, and harvest the preliminary data useful for water quality improvement of agricultural reservoirs, which heavily polluted with organic materials. The use of domestic freshwater mussels, Unio douglasiae and Cipangopaludina chinensis have an advantages such as the strong tolerance to extreme environments, 2) no outbreaks of animal population in one region or area like Zebra mussel, 3) low disturbance to aquatic ecosystem of native animal application, and 4) economic to man and application. Feeding experiments largely comprised 1) the checking of filtration activity on different experimental water (stream and lake), 2) alone and combined effects of both animals Unio douglasiae and Cipangopaludina chinensis on the same prey, 3) feeding activity on both cold- and warm phytoplankton communities of the same lake, and 4) In situ mesocosm at shore of eutrophic lake. Laboratory results indicate that 1) the animal U. douglasiae clearly decreased the diatom biomass (chl-a) more than cyanobacteria of field water, 2) combined treatment with C. chinensis synergistically showed the high removal activity on algal bloom water, 3) grazing effects were more effective on the phytoplankton community of cold-lake water than that of warm lake water. In addition, in situ mesocosm results indicate that 1) the grazing effect of mussels was obvious only for the initial 3-days, but gradually decreased, 2) low density treatment rather increased the nutrient concentration, decreased the dissolved oxygen, and death of mussels, suggesting effects of high temperature above 30 oC, ammonia production, and sediments. Therefore, the field application of domestic freshwater filter-feeder bivalve, U. douglasiae to control the nuisance algal bloom or organic material in eutrophic lake, firstly might be considered a sediment and high temperature of the target reservoir, and can be enhanced by the combined treatment of co-inherent mollusks like C. chinensis, being shared the habit in ecosystem.