The lip prolapse observed in P. mesopotamicusand the preference for the surface waters shown by both species can be indications of low dissolved oxygen in the water. This can be caused by an increase of oxygen consumption by the fish or by the degradation of TFB. Moreover, TFB degradation and consequent release of fluorine and chlorine ions in the water, together with accumulation of excrement from the fish, caused increase in the water electrical conductivity ,which may interfere with the oxygen assimilation, homeostasis, and osmotic regulation in fish.