Describe how excess nitrogen from agriculture can kill fish.

Nitrogen from agriculture can be easily drained to rivers or ponds where the fish lives through processes such as rain or floods.

When these inorganic substances is drained to the fish habitats, algae may grow rapidly on the surface of the water. This is called algae bloom. It's most likely be algae bloom if you notice a big green patch on a water surface.

Algae does not take away all the oxygen of the water and kill the fish. This is what most people thought. But note that algae, just like other green plants, they make oxygen by photosynthesis. The reason why fish are killed is that once the algae dies, which most likely a lot of algae will die at the same time, since they grow in the same time period, decomposers such as bacteria would decompose them. This process is necessary in a cycle.

However, bacteria cannot photosynthesize. Instead, they take in the oxygen for aerobic respiration. This is the real reason why fish dies. They'll be lack of oxygen.