Algae growth inside a garden fountain makes crystal-clear water appear green and dirty. The slime grows fastest with exposure to sunlight and can eventually clog the fountain's filter. Maintenance of a fountain includes cleaning it on a regular basis to remove algae; it also includes treatment to help prevent algae. These measures may not completely inhibit algae growth, but they can reduce it to a more manageable rate of growth.
Removing very light algae growth does not require completely dismantling the fountain and cleaning it thoroughly. In some cases, you can simply wipe the algae from the inside the fountain basin, allowing the water and loose algae debris to run through the fountain filter. Use a cleaning cloth or a sponge with a mild abrasive to wipe away the algae. If the water appears especially dirty after you knock the algae loose, you may wish to replace that water with fresh water. Although this light cleaning routine leaves the fountain basin clean, the algae builds up in the filter, which will eventually require a deeper cleaning.
A garden fountain may require thorough cleaning once every few weeks to few months, depending on its size and preventative measures taken against the algae. Unplug the fountain's filter, and set it aside. Empty the water from the fountain, either by tilting the fountain, siphoning the water or pumping out the water. Scrub the fountain basin with hot, soapy water made with a mild dish detergent. A solution of diluted bleach -- 1 part bleach to 9 parts water -- can help kill and prevent algae. Rinse the fountain thoroughly after cleaning it. Submerge the fountain filter -- not the filter motor -- in hot, soapy water, and scrub it clean with a scrub brush. Use the pressure from running water to dislodge debris stuck on the filter.
Algae is photosynthetic and thrives on sunlight, although it doesn't grow leaves or have roots like plants in a garden. Without sunlight, algae can't grow. So the easiest way to prevent algae in your fountain is to place it in shade. If you don't have a shady corner or location under a shade tree for the fountain, then place it near an arbor or similar structure that blocks sunlight. Impurities in water also lead to algae in a fountain. Therefore, avoid filling your fountain with contaminated water, such as unfiltered water from a pond or stream. Instead, use treated tap water or distilled water from which all impurities were removed.
Garden centers sell a range of algae-prevention products, known as algaecides, for pools, ponds and fountains. These products can kill algae already present in the water or keep it from growing in clean water. Application instructions vary among the products and vary with fountain size, but algaecide is generally used for a fountain by first diluting the product with water -- usually 1 part algaecide to 4 parts water -- and pouring the diluted solution into the fountain at a spot near its pump. The water becomes foamy as the algaecide begins working against algae, then it eventually becomes crystal clear. Household bleach can be an effective algae prevention method when neither plants nor other aquatic life live in the fountain, but bleach is generally not recommended because it can damage the fountain's material.