For the purpose of this trial, triplicate groups of common carp (Cyprinus carpio L.), were distributed in twelve cages of 125 m3 on Tikveš Lake (Republic of Macedonia). Approximately 1000 two-year-old fish, with initial mean weight of 400 g, were randomly distributed in each cage. All fish were reared under variable natural atmospheric conditions. Four isolipidic extruded diets were formulated (FSH Komponenta, Ćuprija, Serbia) to provide 330 g kg−1 protein [high protein (HP) diets] or 280 g kg−1 protein [low protein (LP) diets]. Experimental diets were further prepared by addition of 60 g kg−1 of rapeseed oil (RO) or 60 g kg−1 of fish oil (FO) (Table 1).