The excessive use of one of the following in agriculture can cause the death of fish in a pond by oxygen starvation. This one is:
The fertilizers act as nutrients for the algae to grow. When these algae die, they act as a nutrient for decomposers like bacteria. As a result of this process, a lot of oxygen in the water gets used up and leads to the deficiency of Oxygen in the water. This may kill the aquatic organisms.