Fish die in water bodies polluted by sewage due to

 Oxygen enters the water through diffusion and water has the highest potential to hold oxygen. The high temperature, untreated sewage having biotoxins, hazardous waste can reduce the oxygen to an alarming level leading to the hypoxia in fishes, whereafter enduring at the maximum level they die resultantly. This occurs because the untreated sewage is rich in organic matter and microbes utilize the dissolved oxygen for the decomposition called biological oxygen demand (BOD). The more the value of BOD, the lesser will be the oxygen in the water.