Algae is present in most ponds in varying population levels. It can take the form of filamentous algae---creating strings of growth or dense green mats---or exhibit itself as simply green water if the algae cells are waterborne. Algae becomes both an ornamental problem and a health hazard when it makes your water excessively murky and begins affecting the health of your fish and water plants. Get rid of algae using algae-killing and control techniques.
Remove as much bottom-growing filamentous algae and floating mats of algae as possible. Scrape the bottom of the pond with a rake, and collect floating algae with a net. Pile the algae in the sun to dry it out, then discard it or throw it in your compost pile for a nutrient-rich additive.
Reduce excess nutrients in the water, a common cause for algae blooms and invasive filamentous algae growth. Observe your landscape and ensure there is no water runoff entering your pond, which may carry debris and dissolved fertilizers. Scoop out floating debris such as sticks and leaves.
Plant decorative water plants so that the plants cover 50 to 75 percent of the garden pond's surface. Example plants include water lilies, popular for their colorful blossoms, and the hardy duckweed plant. Adding more water plants reduces the amount of sunlight entering the pond, thus starving the algae, and also reduces the excess nutrients in the water.
Add a pond dye to further lower the amount of sunlight reaching below the surface of the water. This does not affect floating garden plants, but reduces underwater light to starve algae of the sunshine it needs to photosynthesize. An ornamental benefit is the pond dye can turn your pond into an attractive blue shade.
Mix in an aquatic algaecide formulated with copper chelate or copper sulfate and intended for garden pond use. This option should be reserved for heavy algae blooms and infestations. Remove all decorative garden plants before treating the water, and apply the chemicals according to the specific product guidelines as algaecide efficacy varies by manufacturer.
Prevent the algae from growing back by adding barley straw to the water as a preventive step. As barley straw decomposes, the resulting chemical reactions will stop algae from growing. Barley bundles can be obtained at most pond supply stores and should be applied at a rate of 3/10 oz. per square surface yard. Repeat every 6 months to stop new algae growth.