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¹ Trakia University, Faculty of Agriculture, Department of Biology and aquaculture, Students Campus, 6000 Stara Zagora, Bulgaria DOI : 10.4194/1303-2712-v13_1_13 Viewed : 5323 - Downloaded : 5620 Conventional water treatment in recirculation aquaculture systems (RAS) is a limited technology to answer the challenges of so called “sustainable aquaculture”. This is why new and innovative technologies need to be invented and introduced in RAS. The aim of the conducted study was to determine the possible advantages of using two macrophytic plants – Lemna minor and Wolffia arrhiza and their quality as biological filter in a RAS for the cultivation of fingerlings from common carp. The temperature, dissolved oxygen, pH and conductivity were measured daily with a portable combined meter and with a probe appropriate for the parameters in the newly constructed control and experimental RAS (with floating macrophytes as a biological filter). Ammonium, nitrite, nitrate, total nitrogen and phosphorus were measured spectrophotometrically. At the end of the trial the fish were weighed and individual weight gain, specific growth rate and FCR (feed conversion ratio) were calculated. The utilization of two macrophytes (Lemna and Wolffia) in their quality as a biofilter in RAS increased dissolved oxygen in the water, significantly decreased the quantity of total dissolved solids, ammonia, nitrite, orthophosphate as well as total phosphorus in water, and significantly increased the growth of the cultivated carp’s fingerlings. Keywords : Lemna, Wolffia, biofilter, RAS, water quality, carp’s fingerlings