Drain the pond for spring cleaning after you remove the fish. Remove all of the algae from the pond. Sprinkle kosher salt on the walls and floor of the pond and let it set for three days. Sweep as much of the salt as you can into a pile and remove it. Rinse the pond thoroughly before refilling it.
Reduce the amount of sunlight that reaches the bottom of the pond. Algae have to have sunlight to grow. Limiting this essential growth element helps prevent its growth. If possible, dig out the bottom of a small pond to make it deeper using a shovel. For larger ponds, drain the water by digging a drainage trench with a shovel, allow the ground to dry and hire a bulldozer to dredge the bottom of the pond. Another option is to raise the water level. Set the depth of the pond at three or more feet for best results.
Add a coloring agent to the water to provide an inhabitable environment for pond scum. Do not use a coloring agent if there are fish in the pond as it may disrupt the life cycle of the pond.
Cover as much as two-thirds of the pond's surface with floating plants. They provide nutrients for the fish in the pond, yet provide shade on the bottom thus making it hard for algae to grow. Plant water lettuce, parrot's feather, water hyacinth and water lilies.
Keep a bio-filter in the pond. These bacteria help break down the debris that falls into the pond. Without the debris, algae loses its food source.
Clean the pond's surface regularly. Use a skimmer net to remove leaves and grass that fall into the pond.
Provide aeration to the water to add oxygen by using an aerator. This deprives the algae plants of carbon dioxide.
Use an algaecide to kill pond scum. Select one that doesn't harm the fish or plants in the pond.