Pond Filter Choices


A filter is the means by which your koi are kept alive. It is bought to complement the size of a pond and installed at the same time as the pond. The simplest way of obtaining an effective filter for a small to medium pond is to buy a pre-fabricated filter. However, the larger the pond, the more likely you will have to construct your own.

Before deciding how to filter your pond, you have to make a fundamental decision as to whether your filter will be pump-fed or gravity-fed. Generally, classic pump-fed filters are best suited to smaller ponds, with larger ponds requiring a greater turnover of water – something that is best accommodated by a gravity-fed system.

Pump-fed or gravity-fed?

Pump-fed filter. A submersible pump is sited in the pond which pumps water from the pond into a filter which is positioned above the water level. The water runs through the filter and returns to the pond under gravity, often feeding a waterfall. A relatively recent entrant to the pump-fed filter market is the pressurised ‘canister-type’ filter that can be buried in the ground, with the filtered water returning to the pond under pressure.

Gravity-fed: The filter sits alongside the pond and is buried to lie flush with the ground. It is connected to the pond via pipework (usually from a bottom drain and/or a surface skimmer). A pump in the final chamber returns the clean water directly into the pond.

A filter performs several roles as a life-support system.

1. Mechanical filtration. Particulate material that is enters the filter is removed by various means (entrapment, settlement, vortex), allowing the cleaned water to flow into the remainder of the filter.

2. A filter also performs a biological function, detoxifying the poisonous waste products excreted by koi. Beneficial bacteria colonise the hard surfaces of the filter media, digesting and breaking down a host of soluble organic and inorganic compounds.

The biological function of the filter takes weeks or months to become established and during this time, the pond is most vulnerable to poor water quality. Care must be taken to monitor the water quality during this running-in period as koi stress and fatalities will soon occur during this time.

What options does the koi keeper have when choosing a filter and what are the pros and cons of different formats of filter?

Pump-fed filters

1. The black box filter.

(£60 – £350 depending on complexity, media and size)

This is probably the most popular off-the-shelf filter for a small koi pond. As it’s name suggests, it consists of a water header tank filled with various types of filter media ranging from a graduated set of open cell foam sheets, flocor, alfagrog, and other inert biomedia. In it’s favour, the black box filter is easy to install, involving a simple connection at the top to the hose leading from the submersible pond pump. The pumped water is distributed over the media via a spraybar, trickling through the media and returning via gravity out of the bottom of the black box into the pond. The media performs a combined biomechanical function which can lead to maintenance and filter performance issues.

Foam media can be prone to blocking, leading to water by-passing most of the media and returning to the pond unfiltered. They can also give problems when overwintering a pond, as the chilling effect will lead to them being turned off over winter which will then lead to the loss of valuable filter maturity, putting your water quality at risk in the following spring. They are also a challenge to hide as they need to be positioned above the waterline. In my opinion, black box filters come a close second behind pond netting as one of the biggest eyesores in koi keeping.

To overcome the aesthetic problem for the garden pond market, pressurised canister filter have been developed. Although these can be buried out of sight, their capacity for solid waste is very limited and should not be considered as a serious option for a koi pond filter.

2. Bubblebead filters

(£600 – £3000 depending on size and backwashing facility)

These are the external pond filters that look like a giant egg timer. The beads (from which these filters get their name) are the ‘business end’ of the filter and are made from a buoyant polyethylene plastic, each of which is 1/8” in diameter. They achieve both mechanical (solid entrapment) and biological (bacterial breakdown of waste) functions in a similar way to an undergravel filter that has been turned on its head. Where gravel in an undergravel filter serves both mechanical and biological functions on the base of an aquarium, the buoyant beads do exactly the same but in the top of the filter. That is where the similarity ends, as typically, a bead filter is cleaned more frequently (on a weekly basis) where as an undergravel filter could be cleaned once every 3 months!

The beads supply a large surface area (but are not porous, unlike other biological media) attracting a film of bacterial growth within the steady stream of pond water. As the beads accumulate and ‘bed down’ at the apex of the filter, they also form an effective mechanical filter, screening the water of particulate matter down to 20 microns. Finer particles (less than 20 microns) can also become trapped in the sticky bacterial film that coats each bead.

Eventually, the floating bed of the multi-functional beads will become blocked with the debris that they have been designed to intercept, requiring backwashing. The discharged nutrient-rich water can be diverted onto the garden or down a drain while the perforated pipework within the chamber retains all of the beads in the filter. The discharge valve is closed and the pumped water reintroduced from the pond to re-float the beads.

3. Sand pressure filters.

(£350 – £500 depending on size – Does not include the cost of pump required to run it)

Although technically a pump-fed filter, a sand pressure filter can really only be described as a mechanical filter. It is run alongside other forms of filtration to polish the water by removing even the finest of suspended solid particles. These filters run a very thin line between being very effective and to labour intensive of. That is, they require regular backwashing. Such a filter consists of a glass-fibre pressure vessel inside which a bed of silica sand sits on top of a fanned network of porous pipework through which the mechanically filtered water is pumped (or pulled). On account of the pressures required, a sand pressure filter is usually driven by its own dedicated external pump which also doubles as a back washing pump once the flow is diverted through a very ingenious multi-valve system. Sand pressure filters were very fashionable in the late 1980s to the mid-1990s but have been superseded by higher performance self-cleaning units such as the Answer.

Gravity-Fed Filters

(Pre-fab. Multi-chamber filters £250 (2500 gallon) – £1000+ (£6500 gallons))

Gravity-fed systems are the traditional (and most versatile) methods of filtering a koi pond. Off-the-shelf units are available for ponds up to several thousands of gallons in volume, and for even large ponds, multi-chambered filters will be constructed in-situ, being incorporated alongside the pond and built as an integrated extension to the pond itself. The key behind the versatility of a multi-chambered gravity-fed filter system is that it’s modular approach means that it can be designed to compartmentalise the different, yet complementary stages of filtration.

For example, the first chamber(s) will be designed to remove or capture the solid matter from the dirty pond water entering the filter. This might involve a vortex chamber (see later), followed by a larger settlement chamber filled with brushes to slow down further and intercept the water. Subsequent biochambers will have media designed specifically for it’s biological function such as Japanese matting or alfagrog. The final chamber will house a submersible pump (unless an external pump is incorporated into the design) which then pumps clean water back into the pond.

Besides it’s versatility in design and capacity to filter large volumes of water, a gravity-fed filter offers the koi keeper many benefits. The functionality of each specialised chamber can be improved further by adding waste outlets at the bottom. By simply opening the tap to waste, the contents of each chamber can be independently discharged making maintenance very time efficient. Furthermore, the media within a gravity-fed system is always ‘wet’ which means that even when flow rates may be minimal over winter, the priceless filter maturity will be preserved. As a gravity-fed system will be at pond-level, they are far easier to disguise or hide than many pump-fed alternatives.

The ultimate way of feeding a gravity-fed system is via a bottom drain – or several, should the pond’s size require them. These take dirty water from the bottom of the pond and deliver it at slow speed straight into the first mechanical chamber, aiding mechanical filtration. Consequently, to be effective, a gravity-fed filter chamber should be installed and constructed with the pond (as retro-fitting a bottom drain is impossible).

Optional/Supplementary Filtration

1. Vortex.

(£150-£450 depending on unit’s volume and diameter)

A vortex takes simple settlement a stage further by using a number of different flow characteristics to allow debris to settle. A large conical chamber will receive water from the pond in a fashion which causes the water to ‘spin’ within the chamber. The water leaves the chamber through a surface drain position centrally, passing into the next chamber. The spinning action and changes in direction experienced by the water causes solid matter to settle out on the bottom of the chamber.

A vortex is like settlement in that it is better fed by gravity through a bottom drain rather than pumped water. If a pump is used to feed dirty water from a pond to a filter then entrapment is a better option for mechanical filtration.

2. Protein Skimmers

(£200 – £450 depending on size and make)

This is one of the latest filtration innovations for koi ponds and adapts the process of protein skimming that has been used routinely in marine aquaria for many years.

What is Protein Skimming?

Protein skimming is the action by which proteins (and many other soluble pollutants) can be removed from the pond water. Some of these soluble compounds can cause the water to discolour or froth, with their removal reducing these unsightly phenomena. Protein skimming also assists the bio-filtration process by physically removing compounds which would otherwise be processed by a biofilter.

The undesirable soluble contaminants are irresistibly attracted to the bubble surfaces, forming a foam which builds and collects in the skimmer ready to be removed. The more foam that is produced and collected the greater the amount of soluble contaminant removed, reducing the workload of the filter.

This ingenious piece of pond equipment  is not a necessity but can provide additional benefits to the operation of filtration in the provision of good water quality.

3. Vegetable filters

(DIY project ranging from the cost of just the plants to £100 for a fitted and plumbed-in vegetable filter)

A vegetable filter can be constructed as a means of tertiary filtration to remove nitrates and phosphates from pond water. Fast growing and harvestable marginal aquatic plants are required for the effective uptake of these dissolved minerals, with watercress and parrot’s feather being the more popular candidates. A vegetable filter can range from the crude (vegetation growing out of the final chamber) to the more elaborate (growing beds incorporated into a waterfall). Either way, a vegetable filter can be an effective and aesthetic means of keeping on top of nitrates in the warmer months.

Koi pond filtration is a huge topic and it is an impossible task to cover the full complexity of the choices available to the koi keeper today in such a short article, particularly as new models and innovative technology are continually being applied to meet the challenges of filtration. For this reason, I have looked at filters from a generic viewpoint, rather than describing specific makes, models and brands of filter. But by identifying a number of your pond’s fundamental features and requirements you can soon settle on the type, size, design and complexity of filter required for your pond to function optimally for the health and benefit of your koi.

Kill blanketweed and string algae.