Outdoor goldfish ponds are susceptible to algae infestations. Algae can use up the oxygen present in the water, making it difficult for the goldfish to thrive or even to survive. Removing the algae from your goldfish pond will ensure the fish have enough oxygen. You can take several steps to remove the algae, all of which involve removing the algae's sunlight and nutrient sources.
Install shade-producing water plants in your pond, such as water lilies or water hyacinths. The plants will block sunlight from reaching the water and encouraging algae growth. If you do not want to install plants in your pond, introduce a sun-blocking chemical additive to the water according to the package directions.
Remove any leaf litter or other debris from the bottom of the pond with a rake or pond vacuum. Decaying leaves and twigs that settle on the bottom of your pond provide nutrients and a growing medium for algae.
Install a pond skimmer onto your filtration system. A pond skimmer will filter out leaves and other types of small debris before they settle on your pond floor. Follow the manufacturer's guidelines for installation.
Drop in one application of beneficial goldfish pond bacteria, available from your local pond-supply store, according to the package directions. The beneficial pond bacteria will consume the nutrients that keep the algae alive, causing the algae to starve and die without harming the goldfish. As the algae dies, it may settle on the bottom of the pond. Check the pond daily for green build-up of dead algae and remove it with a pond vacuum.
Install a pond aerator system. An aerator will mix the water and inject oxygen into the deeper areas of the pond, encouraging the growth of the beneficial bacteria and your water plants. Purchase an aerator that is appropriate for the size of your goldfish pond.
Avoid overfeeding your goldfish. Both the uneaten food and the waste the fish produce provide nutrients for algae growth. Also, avoid using lawn and garden fertilisers on any area uphill from the pond. These also contain nutrients that can cause algae bloom and water from rain will wash the chemicals into the pond.