While some algae in the backyard pond will help keep the water oxygenated and the fish healthy, when algae grows out of control it can definitely be a problem. It can block sunlight and strangle plants, throwing off the balance of the entire pond ecosystem. Several different methods can be used to control algae, and depending on the severity of the problem, it may be necessary to use one or all of the methods. Some work as preventatives as well as cures.
Add some floating plants, such as water lilies, which will block some of the sunlight that reaches the pond, and will also consume some of the nutrients that are in the pond water. The more sunlight that penetrates the water and the more nutrients that are available, the faster algae will grow. By using floating plants to consume some of these resources, there will be less to feed the algae.
Raise the banks of the pond to prevent runoff from entering the system. This runoff can be particularly problematic if there are gardens nearby that are fertilized regularly, as this fertilizer can be transferred into the pond water. Once there, this water pollution will fuel the growth of algae; on a large scale, fertilizer runoff can be responsible for blooms that devastate pond ecosystems.
Add some pond dye. Pond dye will change the color of the water without affecting the plants or animals. As with any chemical, it is important to check with a professional first to be sure the dye you've selected is safe for the pond life you have. Dying the water will limit the amount of sunlight that is allowed into the system and increase what is reflected, providing less for algae to feed on.
Be sure the filter is working properly and is cleaned regularly, or install a new filtration system if the current one is old, outdated or not operating at its peak potential. A healthy filtration system will remove much of the algae before it has a chance to reproduce.
Limit the nutrients that are introduced to the system. If there are fish in the pond and they are fed commercial foods on a regular basis, any uneaten food will dissolve and become fuel for algae.
Apply algaecides. This is usually only done as a last resort, as this control method only lasts as long as the algaecide is present in the water. These chemicals can also damage other plants in the pond, and may be toxic to fish and other animals. There are some algaecides that are marketed as safe for plant life, but if there are other options, using an algaecide should be avoided.