How may nitrate from fertiliser entering lakes cause the death of fish in fresh water?

The addition of excess nitrate to the freshwater body causes algal bloom on the surface, blocking sunlight. This reduces the photosynthetic activity of submerged plants, causing them to die. Subsequently, they are decomposed by saprobiontic microorganisms, which use up the oxygen in the water via the process of aerobic respiration. This means that the water becomes anoxic, and aerobic organisms like fish also die.