Fertilizers are applied to fishponds to serve as nutrients for plankton. Plankton are tiny organisms that live in water and are a primary source of food for fish. Fertilizers contain essential nutrients such as nitrogen and phosphorus that are necessary for the growth and reproduction of plankton. As the plankton grow and multiply, they provide a valuable food source for fish in the pond. Additionally, the plankton consume nutrients in the water, which can help purify it by reducing the levels of harmful chemicals and pollutants. Therefore, applying fertilizers to fishponds can help promote the growth of plankton and ultimately support a healthy ecosystem for fish.