Fish change their energy metabolism, spending a larger quantity of energy to mitigate the stress. This paper aims is to investigate the changes of some important respiratory indices (energy metabolism, respiratory rate, number of red blood cells) for three common species in the Arges river (Carassius gibelio Bloch, Perca fluviatilis L. and Alburnus alburnus L.), exposed to different conditions of temperature and degree of water hardness. The samples of fishes were being investigated at two temperature levels (18-20ºC, and 6-8°C) and two levels of water hardness (150 and 300 mg CaCl2/l water). The experimental samples regarded the presence of respiratory and hematological changes (and, where appropriate, the extent of these changes) in prussian carp, perch and bleak adapted to different temperatures (6-8°C and 18-20°C) and revealed differences in the reactivity of the three species to changing environmental conditions. Although adaptation to low temperatures caused metabolic decreases and increases in the respiratory rate in all three species, the evolution of these parameters fitted the picture described in the specialized studies. The perch had the lowest oxygen consumption and adapted to low temperatures better than the prussian carp and the bleak.