After nutrient enrichment from sewage contamination, a lake often becomes inhospitable to fish. why?

Due to lack of oxygen concentration.

Explanation:

Nutrients intake in water bodies leads to growth of algal bloom and more algal bloom depletes the level of oxygen in water and makes it unsuitable for the other aquatic organisms to survive there.

After sometime, cyanobacterial population and algal bloom is decomposed by the bacteria present in water, which utilizes the oxygen present in water.

Less than required amount of oxygen in water leads to death of fishes.