Overgrowth of algae is caused by excess potassium and other excess nutrients which promote a higher rate of photosynthesis for the algae. This is why excessive runoff from fertilizers is harmful because it can cause overgrowth of algae. The overgrowth of algae spreads throughout the lake and blocks sunlight from reaching inside the body of water. When no sunlight reaches, the plants at the bottom of the lake cannot photosynthesize and therefore, die off. There is no oxygen for the fish therefore the fish die out.