The role of a filter in your water garden is simple: It keeps the water clear and free from debris.
But a filter needs water pulled or pushed through it, so nearly all water garden filters work in conjunction with a pump.
Some pumps come with filters attached. Some pumps require a filter that is purchased separately. And larger water gardens may require more than one filter to keep the water clear and healthy for fish. Just like in an aquarium, fish produce an enormous amount of waste that pollutes the water if it isn’t removed by a filter or by plants.
There are different types of filters.
A prefilter is exactly what it sounds like: A filter that filters out debris before (“pre”) the pump to prevent bits of plants and algae from clogging the pump.
Prefilters are often built right into the pump. It may be a piece of foam that fits onto the end of the pump. Or it may be a slatted snap-on encasement. With larger pumps, it may be a separate mechanism that rests alongside the pump, attached with tubing.
This type of filter is basically just a very good strainer. Water runs through it, pushed or pulled by the pump. Mechanical filters can be made of foam, netting, grates, or screening to catch particles in the water.
Mechanical filters need regular rinsing out. Depending on the size of your water garden, the filter, and the type of water garden you have, that can be every few days or just a few times each growing season. Usually cleaning it means just lifting it out of the pond, snapping it apart, and giving it a good hard spray with the garden hose.
With smaller pumps, the most common type of mechanical filter is about the size and shape of a shoebox. It contains layers of foam to filter out debris, and a lid with lots of little holes or slats in it to filter out larger pieces before they are pulled through the foam.
A skimmer (see below) is also a type of mechanical filter.
While prefilters and mechanical filters remove physical bits and clumps from the water, biological filters work by housing helpful bacteria that keep the water clear and healthy.
Biological filters come in many different styles. Most consist of a media of plastic, rock or ceramic in which the helpful bacteria live, often held together in a mesh bag. Biological filters also may contain helpful enzymes.
Biological filters are stashed somewhere in the pond—often in the small pool (also called a weir) at the head of a waterfall. The water flows through the biological filter and is dispersed through the water feature.
As with other filters, you must clean biological filters. Depending on conditions, that could be once a month or once a year.
UV Filters or Clarifiers
The high-tech way to keep water clear is with a UV filter. These usually come in a tube-like container equipped with an ultraviolet light. The container can rest in the pond or can sit alongside the pond, with tubing feeding water to it.
Water is pulled or pushed through the container. The water contains problem-causing single-cell organisms, such as algae, fish parasites, and harmful bacteria. As the water passes into the container, the organisms are exposed to the UV and their DNA is altered. They then pass out of the container and back into the pool or pond, where they eventually die.
UV clarifiers are an especially effective method of controlling floating algae. But they’re discouraged by some water gardening professionals, who are concerned that helpful bacteria and microorganisms are killed alongside the problem organisms. They would rather see gardeners controlling algae by keeping the water ecologically balanced through the use of bio and mechanical filters, plants to shade the water and filter out impurities, and general good design and maintenance.
These are popular, tub-like filters that are positioned alongside a pond. Their lip is placed just below the water surface, so that water constantly flows into them. The water then passes through a grate or strainer, that catches leaves and other large debris, and then back out into the pond.
You then open the lid of the strainer (often made of gray plastic to resemble a flat, rectangular rock) and lift out the grate or strainer to easily remove the leaves and debris.