Information provided by TetraPond
The first, essential step to control the algae is effective filtration. Mechanical and biological filters remove impurities from the water, helping to preserve aquatic life and enhance the pond's eco-system. You can reduce maintenance by choosing a pressure filter with mechanical and biological filtration in one unit, or up the ante with one that also incorporates a UV clarifier.
Green water does not necessarily mean a dirty pond. If your filtration system is working properly and your water quality tests normal, you'll need a UV clarifier to combat green water. Clarifiers use ultraviolet light to destroy the reproductive ability of microscopic suspended algae, which is so small it is, in effect, passing through your filtration system and re-circulating.
Dead algae will clump together into particles large enough to be removed by filtration, leaving pond water cleaner and clearer. For optimal performance, change your UV bulb every 11 months of continuous use.