Natural Effective Algae Control for Ponds

As the water warms up and the days get longer algae problems tend to rear their ugly heads. Algae is caused by a combination of available nutrients suspended in the water column (nitrate, phosphate) and the abundance of sunlight hitting the pond surface. There are various ways to address algae blooms within a pond. 

The first is to decrease the amount of available nutrients to the algae. This can be accomplished by water changes where we remove 20-30% of the pond water and replace it with fresh water, repeating this daily until the nitrates and phosphates return to close to zero. However, this can be tricky to accomplish in ponds with high stocking densities or large water volumes. A great alternative plan is to out-compete the algae for the nutrients by establishing what is commonly called a "bog filter" or a planted bog. Bog filters are not what you consider a traditional filter. Instead of using filter media or sponges, bog filters utilize live, marginal plants (plants that naturally inhabit the shorelines of waterways) to use up the nutrients before algae can. There are many different ways to incorporate a planted bog into your pond. Many can be found by Google searching "Bog Filter". The easiest is to submerge large numbers of bog plants in basket pots filled with pea gravel in a shallow area of the pond where there is plenty of water movement and sunlight. Make sure not to place the plants too deeply into the pond. The top of root ball should just be under the water's surface. If you purchase wa marginal plants from a garden center, rinse the potting soil off the root ball thoroughly or the soil will cause your pond water to become discolored. Even with a bog filter you should still change 20-30% of your pond water by actively removing and replacing it at least once a month. 
The second way to decrease algae in your pond is to add an in-line UV sterilizer to your filter system. These fit into the plumbing as it makes its way back from the filter into the pond. These will kill the floating, suspended algae as it moves through the unit. However, if your pond is highly stocked, gets lots of light, and doesn't have many other plants, UV sterilizers often fail when used alone. 
The final way to help decrease algae blooms is to decrease the amount of sunlight hitting the pond. This can be done using either artificial or live floating plants. Often in ponds with ravenous or large numbers of Koi you will want to start with a good number of mature water lilies if you choose live plants. Pick a hardy variety that reproduces quickly and can handle some stress from fish feeding on them. A good choice is the Mexican Yellow Hardy Water Lily. 
For more information on water gardening and plants we recommend the book The Encyclopedia of Water Gardening.