Remove the water from the fountain. If the fountain is light enough, try tipping it to allow the water to spill out. If the fountain is heavy or impossible to move, siphon the water out with a garden hose.
Scrub the fountain with a clean scrub brush to loosen any adhered algae. Rinse the loose algae from the fountain with a garden hose.
Refill the fountain with water from the garden hose. Empty and refill the fountain every two to three days to prevent new algae from forming.