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Water Gardens: Biofiltration

by Nellie Neal

Filtering the water in pond gardens keeps the water clear and is essential to a healthy pond unless you have a nearly ideal balance of plants and fish. Most gardeners begin with a mechanical filter system, which works well but must be cleaned frequently in hot weather. More and more are turning to biofiltration as an alternative, especially where warm water encourages algae to grow and in gardens that include fish like koi.

Biofiltration works like mechanical filters. Both remove floating particles of silt from the water and foster the growth of beneficial bacteria that degrade toxic ammonia in the pond into harmless nitrates. But biofilters need cleaning only once or twice a month and it’s a simpler task to backflush the filter with your garden hose.

The newest biofilters stand outside the pond, but are easy to camouflage with plants or fencing. The water from the pond flows through an aeration tower into the tank, where it filters through gravel or mats loaded with bacteria. The clean water returns to the pond. Any muck left in the process falls to the bottom of the tank where a valve can be opened to clean it as needed.

Although biofilters may be more expensive to purchase, they deliver great value over the life of your pond. Once the system is in place, only an occasional purchase of bacteria will be necessary to supplement those that grow naturally. Not only do they last longer and need less cleaning, many gardeners find biofiltration delivers cleaner water.

When converting from a mechanical system to biofiltration, you may need to slow the rate of water flow to the filter. The bacteria work more slowly to change the ammonia first to nitrite and then to nitrate. It is a natural decomposition that continues so long as the bacteria are present. Adjust pumps to allow the pond water to recirculate once every four to six hours.



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