A pond may be green due to the growth of algae or duckweed, which both thrive as a result of high temperatures, a lack of rain and excessive fertilizer use. However, duckweed is important to a healthy pond as it is food for fish and birds, helps reduce evaporation, and controls the breeding of mosquitoes. The most effective way to control algae is to reduce or eliminate lawn fertilizers. Fountains are not effective in reducing algae as they become blocked with pond vegetation. Chemicals cannot be used as they would have a negative impact on surrounding watercourse habitats.