Water Clarity vs. Water Quality
by: Demi Fortuna
Filtration is one of the most misunderstood components of the backyard pond, mostly because of the confusion between water CLARITY and water QUALITY. The most toxic compounds in the fish pond, ammonia and nitrites, are invisible waste byproducts of the fish themselves. They are present in every pond with fish in it, and affect the fish in concentrations of as little as one or two parts per million (ppm). The water can be crystal clear and still be absolutely lethal to fish. The reverse is also true; the water can be murky, completely hiding everything in it from view, and still be wonderfully healthy for fish. In fact, the farmers that produce the finest Koi in the world can't see a single fish in the natural ponds they grow them in until the ponds are drained.
Two types of filtration handle the two different tasks. Mechanical filtration addresses Water Clarity; biological filtration handles Water Quality. Mechanical filtration separates particles from the water, by screening, settling or trapping the particles in filter media like a pad or a mat. Biological filtration harnesses the power of Mother Nature to remove invisible toxic chemicals like the ammonia and nitrites mentioned above. Specific strains of naturally occurring bacteria "eat" ammonia and nitrites, turning them into nitrates, or plant food.
The nitrates get taken up and removed by plants to complete the biological filtration cycle, also known as the Nitrogen Cycle. Biological filters provide a place for these bacteria to grow and an aerated supply of pond water to clean. Both types of filtration, mechanical and biological, are needed for crystal clear, perfectly healthy water.
In large natural ponds, solids and sediments are screened out by vast multi-species beds of roots and submerged foliage, and settle out in still waters. Entire populations of fish, plants and bacteria interact to balance and maintain healthy water quality.
In the backyard pond balance is more difficult to achieve because even small shifts in temperature, pH or organic input (food!) can strongly affect the relatively tiny ecosystem of the water feature. These ponds greatly benefit from by well-designed filtration equipment that optimizes these natural processes and compensates for the lack of real estate.