Filters to clean pond water

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Last updated 27th June 2014

Why filters are essential for some ponds
Types of pond filter
Mechanical filters keep pond pumps working
Biological filters keep pond fish healthy
Biological filters for goldfish ponds
Biological filters for Koi ponds
Maintaining pond filters
Natural ponds – optional filters
Alternative and additional pond filters
Eight top tips for pond filter choice and set-up for fishponds

Why filters are essential for some ponds

There are two possible types of impurity in garden ponds – visible debris and invisible chemical compounds.

  1. The debris is a nuisance because it can damage pumps, obscure your view of pond inhabitants, clog up fountains and settle on every surface to give an unsightly coating.
  2. The invisible impurities from decaying plant and animal matter, plus fish excrement, are far more insidious for fish. In the case of Koi ponds, which are generally stocked with very high levels of livestock in a plantless pond, it doesn’t take long for toxic levels of pollution to be reached.

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Types of pond filter

To deal with these problems there are:

  • mechanical filters – these act as coarse sieves, straining out debris before it can damage equipment.
  • biological filters – these provide a home for bacteria to thrive on the fish-toxic chemicals and convert them to more benign substances.

Read our guide to different types below, and/or visit useful overviews by online water garden suppliers Bradshaws Direct and
Pondkeeper.

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Mechanical filters keep pond pumps working

For ponds with fountains and waterfalls, an effective pre-pump filter will protect the pump and fountain jets. Because this strainer traps any coarse debris, it removes suspended solids from the water. Eventually the strainer will clog up and reduce the water flow, so you have to clean it regularly. This could be as often as weekly if the water is very dirty.

Most sumbersible pumps are supplied with a strainer, but this can be replaced with a more effective version, such as a larger foam block, assuming it will fit your particular pump. However, this is not necessary if you use a filter pump i.e. one that’s designed to feed a filter, which can handle solids better (e.g. from Bradshaws Direct or Pondkeeper), or a self-cleaning type (eg Oase Filtomatic Intelligent Filter from Bradshaws Direct or Pondkeeper) this isn’t necessary.

Alternatively, matched pump and filter sets (from Bradshaws Direct and Pondkeeper) and all-in-one units that usually include a UV clarifier (UVC) too (from Bradshaws Direct or Pondkeeper) make choosing and installing quite simple.

For fishponds, there are options that can be used singly or in combination to clear the large amount of particles found. Choose a vortex-style prefilter (from Bradshaws Direct or Pondkeeper) or multi-chamber filters (from Pondkeeper) containing a variety of mechancial filters such as brushes, mesh, foam, Japanese matting or a mixture. These mechanical filters are sometimes combined with a biological system (see below).

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Biological filters keep pond fish healthy

If your pond contains fish you will need to install a filter system to deal with the chemical content of the water. These systems are mini-sewage works, containing substances called media, which have large surface areas for beneficial bacteria to colonise.

Additionally, nitrites and ammonia can be absorbed by zeolite chippings (from Koi specialists such as Absolute Koi), which also remove odours and discolouration from the water.

Usually the pond water has to flow first through a mechanical filter, which can be made up of brushes, mesh, foam, Japanese matting, Alfagrog, Flocor, Matala or Biomaze (e.g. from Absolute Koi), so that debris and suspended solids are removed.

There are numerous off-the-shelf filter systems available in modular form for a customised set-up, so unless you’re on a very tight budget, you’ve no need for DIY. Some of these (from Bradshaws Direct and Pondkeeper) are made specifically to cope with the high waste levels generated by Koi; for less demanding fish ponds, browse other more compact systems (often termed box filters) from these suppliers. With all of these filter systems there’s often a version with an integral UV clarifier (UVC) unit to deal with green water problems too.

Whichever filter system you choose, be sure to select one that’s easily accessible for cleaning and maintenance. And if you want to hide the filter by burying most of it to make it as unobtrusive as possible, choose one with high level inlet and outlet pipes.

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Biological filters for goldfish ponds

For goldfish, inexpensive single-chamber biological systems are sufficient. They are usually pump-fed, relying on gravity to return the clean water to the pond, so the outlet must be higher than the pond’s surface.

There are also compact pressurised systems where the outlet doesn’t have to be above the water’s surface, so it’s easier to find an unobtrusive place for the unit. Pressurised filter units are made by Fishmate, Hozelock Cyprio, Laguna, Oase and PondXpert, to name a few widely available brands, which you can browse at Bradshaws Direct or Pondkeeper.

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Biological filters for Koi ponds

Multi-chamber systems can cope with the high levels of waste generated by Koi, and are essential to maintain good water quality.

These systems need to include an area for silt to settle, instead of clogging up the media, and a way of flushing this out at intervals, such as those from Hozelock Cyprio’s Trinamic range.

Some systems are pump fed, others are gravity fed and need installing with the water level in the filter at the same level as the pond water. A pump submerged in the last chamber, or fitted externally, sends clean water back to the pond.

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Maintaining pond filters

Mechanical filters need to be flushed regularly (with pond water) to avoid the flow slowing down and the filter clogging up.

Biological filter material should also be washed with pond water, so that any silt is removed but bacterial colonies are left unharmed.

Zeolite chippings need re-charging periodically by soaking in a strong salt solution for 24 hours before washing and drying.

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Natural ponds – optional filters

If your garden pond is a natural-type pond without fish or fountains, then you do not need a filter. There isn’t a pump or jets to clog and no fish to nurture. After some time this sort of pond will settle down of its own accord with all the inhabitants thriving.

Any turbidity due to green water should be temporary if you grow enough plants to block sunlight, and any silt being stirred up by frogs or birds can be siphoned out. If soil is being disturbed in submerged planters, just top them with chippings to keep the soil in place.

If you have fish in your natural pond, create a natural vegetation filter by growing fast-growing plants (such as reeds) in a gravel bed that is level with the pond’s surface. Or pump the pond water through the gravel bed. All the nitrates from fish pollution are used in plant growth, rather than overloading the pond.

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Alternative and additional pond filters

Plant filters: attractive and useful for removing nitrates from water. Trickle returned water through an area of water plants as an additional final filter. Suitable plants include arrowhead (Sagittaria sagittifolia or S. japonica), brooklime (Veronica beccabunga) and watercress (Rorippa nasturtium-aquaticum, syn. Nasturtium officinale), which are best planted with a top dressing of gravel to avoid soil washing into the pond. Planting wildflower plants suited to boggy ground (from Crocus as plugs) is another option that would give a colourful display too.

Sand filters: fitted after the biological filter, the sand filter removes any fine organic debris that has found its way through the system, but does require backwashing every other day so you may not want this extra maintenance. Some Koi keepers swear by sand filters; others at them. Try searching the different forums to discover more.

Fluid bed filters: ideal for fish ponds, fluidised sand beds (eg from Absolute Koi) take up little space, but provide a huge surface area for bacterial colonisation. They can be added when you outgrow your current setup, or installed as a complete system when constructing your pond.

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8 top tips for pond filter choice and set-up for fishponds

  • Size: decide on fish stocking levels and allow for growth when choosing an appropriate filter system. An oversized filter is better than an undersized one. Consult suppliers or the technical departments of manufacturers if you need advice on type and size.
  • DIY: if constructing your own filter system, use inert materials for the filter chambers and fit a valve to the base of each chamber so you can drain off debris.
  • Area: the approximate water surface area of filters should be around 25 per cent of the pond surface area.
  • Flow: aim to circulate all the pond water once every two hours, but make sure that the flow is slow enough for sufficient treatment to occur.
  • Pre-filter for chamber set-ups: a vortex pre-filter removes suspended particles before the water enters the filter chambers, for less clogging and cleaning.
  • Media: choose media that enable a good flow of water, such as plastic mouldings. Direct water alongside materials such as Japanese matting or reticulated foam.
  • Starter bacteria: use a filter starter to introduce essential bacteria to a new set-up, or one that’s been out of action for a while.
  • Cleaning: don’t clean biological filters too thoroughly. Rinse media in pond water and flush away silt but don’t scrub, otherwise you’ll disturb the colonies of beneficial bacteria.

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