Almost any stagnant water, such as a pond, will eventually become clogged with algae, especially during warm weather and when excessive amounts of nitrogen or phosphorous are present. The key to controlling algae growth in a pond in the long run is to remove the source of nutrients that the algae need to survive and to thrive. While chemical herbicide solutions may seem the simplest, most have side effects in that they also kill plants that you want in your pond. If the nutrients that algae need to grow are kept out of your pond, there should be no need to add any chemicals to the water.
Remove the source of nutrients that the algae need to thrive. If water from the lawn or garden is entering the pond, create berms around the pond to prevent this. Runoff from lawns and gardens often contains fertilizers, which contribute to algae growth.
Remove most fish from the pond and reduce feeding until the algae problem is resolved. The nutrients in fish excrement and in uneaten food can provide the necessary fuel algae needs to grow.
Add a pump or fountain to increase water movement in the pond. This will reduce the likelihood of algae growth. Algae prefers stagnant water.
Use a hard rake to rake algae off the top of the pond and to remove as much algae from the bottom of the pond as possible. Dispose of the algae far away from the pond, allowing the algae to dry completely.
Remove all tree leaves and other vegetative waste in your pond as these contribute nitrogen to the water as they decay. This includes removing leaves and debris from the bottom of the pond, as well as from the top of the water.
Use a pool skimmer to remove any last traces of algae.