Aquatics Unlimited: Articles: Pond Filters
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Pond Filters

What Pond Filters Do

Pond filters can serve a variety of functions, including:

  • protecting the pump from debris
  • breaking down dangerous fish waste
  • trapping some suspended particles from the water
  • removing some dissolved waste from the water
  • reducing algae growth by removal of nutrients

What Pond Filters Don’t Do

Unfortunately, what typical pond filters aren’t designed to do is to single-handedly keep water “crystal clear” - at least not to the extent that aquarium filters do. In fact, a filter adequate to keep even a small, 200 gallon pond as clean as an aquarium would have to push about 1500 gallons per hour and use a filter cartridge approximately four feet by six feet!

Types of Pond Filters

Intake Filters

Intake Filters are usually small (perhaps 3x3x6 inches or so) blocks of open-celled foam that often come in the box with the pump. These are far too small to be effective at waste removal or reduction, and serve only to prevent pieces of debris from being sucked into the pump’s intake and stopping or damaging the pump itself. Advantages: inexpensive; easily removed and rinsed clean; output of pump can still be used to run fountain or waterfall. Disadvantages: small surface area means they plug with debris quickly; often need to be rinsed daily or even more frequently; do not improve water quality significantly. Things to Look For: very coarse foam for good flow; firm attachment to pump; multiple filtering surfaces so a single leaf cannot block flow.

Submersed Filters

Lagunas Powerflo submersible filter
Laguna's "Powerflo" submersible filter

Submersed Filters are generally larger and more versatile than intake filters. A reasonable unit for a small pond might have a surface area of a square foot or more, and may use a variety of filter media including coarse and fine pads for mechanical filtration, carbon or zeolite for chemical filtration, and perhaps some sort of plastic or porous stone media for biological filtration. Many units can be installed on either intake or output side of the pump, but are most commonly connected to the intake where it protects the pump as well as cleans the water. Advantages: improve water quality as well as protecting pump; may go days to weeks between cleaning; optional filter media may be added for special purposes; pump output may still be directed to water features. Disadvantages: pondkeeper may need to wade into pond to retrieve filter for cleaning; much waste from filter may spill back into pond during cleaning; take up valuable space inside pond. Things to Look For: easy disconnection from pump; large surface area; room for several different media; filter media compartment recessed enough that waste isn’t spilled when removing from pond.

In-Line Filters

PondFiltrations PF300 in-line filter
PondFiltration's "PF300" in-line filter

In-Line Filters are resealable canisters that may be filled with a variety of media for mechanical, chemical and/or biological filtration. They may be installed on either the intake or the output side of the pump, although most commonly used on the output side. They may also be operated either submerged or above ground. Since they are sealed and operate under pressure, water can be directed from their output to a fountain or water run. Advantages: improve water quality; protect pump if used on intake side; may go days to weeks between cleaning; optional filter media may be added for special purposes; pump output may still be directed to water features. Disadvantages: pondkeeper may need to wade into pond to retrieve filter for cleaning; units may be difficult to open or reseal during cleaning; take up valuable space inside pond. Things to Look For: reliable seal and closing mechanism; easy disconnection from pump; large surface area; room for several different media; heavy duty construction for high pressure when filter or outlet clogs.

External Filters

Tetras ClearChoice external filter
Tetra's "ClearChoice" external filter

External Filters are generally the largest of pond filters, and are located above ground (some may be partially buried) somewhere near the pond itself. Water from the pump’s output enters the top of the filter, trickles through the filter media, then drains out the bottom port. External filters may be operated with a variety of media, but generally use large sheets of foam for mechanical filtration with several gallons of plastic or porous stone biological media underneath. Many operate on the “wet/dry” principle used on high-tech saltwater “reef” aquariums, in which the biomedia has water continuously running over it but is not submerged. Advantages: improve water quality; may go days to weeks or even months between cleaning; most efficient form of biological filtration supports greatest fish load; optional filter media may be added for special purposes; above ground installation makes unit easy to clean. Disadvantages: large size and need to be above water level make them difficult to hide; water leaving filter must drain freely into pond and may not be used for fountain or waterfall. Things to Look For: internal bypass that allows water to continue to flow if filter clogs; large surface area; room for several different media.

Filters May Be Used in Combination

Filters may be used in combination to take advantage of each filter’s strong points. A common setup is to attach a submersed filter on the intake side of the pump, filled with coarse media to prefilter and protect the pump, and also an external filter on the output side, filled with finer media to catch smaller particles and biomedia to improve water quality by reducing ammonia and nitrites. Such a complete system not only provides the best filtration, but also can result in longer times between cleanings.

Sizing the Filters

Regardless of the pond size, a small filter is better than none, a medium better than small, and large better then medium. The bigger a filter is, the more waste it can catch or process between cleh or process between cleanings, and the more fish can be safely maintained The trade-offs of course are that bigger filters cost more and take up more space. Another concern is to match the filter to the pump’s output. An undersized filter will either restrict the water flow, or worse, leak water from the system. In such cases, a bypass should be installed to divert water to another water feature, or better still to another filter.


Copyright © 1998 James M. Kostich
All rights reserved.

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