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Choosing the Best Pond Filter System

With a multitude of different pond filters available on the market today, choosing the best pond filter system for your pond can seem like a daunting prospect. With so many different types, sizes and brands of pond filters available, it can be confusing trying to determine which pond filter is most appropriate for your needs. However, choosing the best pond filter system doesn’t have to be difficult. You only need a few key pieces of information to point you in the direction of the right filter. Let’s take a look at what you need to know to correctly size a pond filter, the different types of pond filters available, and the situations most favourable in which to use them.

How do I Choose a Pond Filter?

When it comes to selecting the correct pond filter system, there are some things you need to take into consideration to ensure that you choose the best pond filter system for your pond. Pond volume, fish stock, pond and pond filter location are key to this decision, so these should be determined from the outset.

1.  The volume of water the pond will hold.

You will first need to know how much water the pond will hold at maximum capacity. To work out the volume of your pond you will need to multiply the Length x Width x Depth, all in metres and multiply the figure by 1000. This will give you the volume of water in litres, which is the best starting point when it comes to choosing the best pond filter system.

2.  The type of fish you intend to keep.

The size of the pond filter you need also depends on what you will be keeping in the pond as that influences how frequently the water needs to be circulated through the filter. A pond without any fish requires a full volume circulation (all the water passing through the filter) less frequently than a pond containing fish, which means a lower flow rate through the filter system is required. A pond with a stock of large Koi Carp will require a full volume circulation more frequently than a pond with only Goldfish, so your fish stocking situation should be decided upon before a pond filter is selected.

Fish Ponds

Any reputable manufacturer of pond filters will provide information detailing what size/volume of pond their product is capable of supporting with a predetermined stock of fish, so with the pond volume and intended fish stock known, a suitable filter can be selected. However, the manufacturer’s specification should only be used as a guide to set you on the right path, as other factors need to be considered.

3.  The location of the pond filter.

It is important to consider where and how you can install the pond filter, as some filters, such as flow-through filters, can only be installed above the pond, whereas you can be more flexible about where you place pressurised filters. Any limiting factors with the installation may prevent the use of a certain type of filter in favour of another more appropriate design that will work for you.

4.  The location of the pond.

If the pond is especially shallow and/or exposed to direct sunlight for large periods of the day, choosing a pond filter system that is above your specifications can be beneficial to overcome those unfavourable conditions – you cannot realistically over-filter a pond, so being a little over the manufacturer’s recommended specification with your filter selection can only work in your favour. Never purchase a filter system that is underequipped for your pond, as it won’t keep up with the demands of the pond and will result in wasted time and money when you need to purchase a more appropriate filter system.


What Pond Filter Type is Best?

For raised ponds, pressurised filters are often the best type of pond filter to use as they offer a more flexible and discreet installation and can be installed below the water level of the pond with the outlet elevated. Flow-through filters can be less flexible, often less discreet, but are less costly to run as this type of filter is more economical from a pumping perspective, requiring only half of the pumping capacity of an equivalent pressure filter. For very small ponds,in-pond filters are a popular choice, as these can be installed directly in the pond with no pipework leading in or out. Ultimately, the style of the pond dictates what the best pond filter is.

Inpond Filter    Pressure Filter    Flow Through Filter

Here are a few of the more common filter types that can be considered:

In Pond Filters

In-pond filters are excellent pond filters if used within their limitations. These pond filters are better suited to small ponds with small fish, offering a very neat solution without the need for any pipework as the whole filter is placed in the pond. Simply remove them from the water when you want to clean and maintain them.

Filtral UVC


In Pond Filters are Ideal for:

  • Small Ponds
  • Raised Ponds
  • Wildlife Ponds

Pressurised Filters

Pressurised filters are watertight units that maintain pressure right the way to the end of the outlet pipe. This type of filter offers certain advantages over the traditional box type pond filter and can be used for most small and medium sized ponds. A pressurised filter can be installed just about anywhere relative to the pond, as the return water can be sent in any direction from the outlet – uphill, downhill or in a straight line, something that is not possible with a flow through filter that returns only by gravity. Pressurised pond filters can also be partially buried in the ground and a good pressurised filter can be cleaned without the need to open the filter up.

BioPress Pressurised Pond Filter     FiltoClear Pressurised Pond Filter

The OASE FiltoClear is a highly developed pressurised filter system. The best pond filter with UV light in the UK features a multidirectional outlet which enables the outlet water to be redirected away from the pond during cleaning. There are 4 models in the OASE FiltoClear range, all of which feature an integrated UV clarifier and an ingenious foam compression system that allows the internal filter foams to be compressed with ease using the top cleaning handle.


Pressure Filters are Ideal for:

  • Small and Medium Ponds
  • Raised Ponds
  • Fish Ponds

Flow Through Filters

Flow-through filters work on a strict principle of pump fed supply and gravity return. This type of pond filtration can be applied from small to very large ponds and is the most simple and efficient means of filtering pond water. However, the specificity of the flow-through principle does restrict what is possible from an installation perspective. As a rule, the filter outlet pipe is a greater size than the inlet pipe (necessary to allow an unrestricted overflow of water from the pond filter), and the pond filter can only be installed above the water level of the pond to allow the gravity return and for the filter box to be drained down for maintenance.

BioSmart 14000 Pond Filter Set    BioTec ScreenMatic 90000 Pond Filter

If the situation allows for a flow-through filter, this is the filter setup that should be used, with other options considered only if installation prevents the flow through solution. Flow through pond filters can be very basic, or very sophisticated, there are many product options available. The OASE BioSmart range is a very good all-round flow through pond filter, for medium size and larger ponds the OASE BioTec ScreenMatic offers a few more technical advantages, with some cleaning automation features.


Flow Through Filters are Ideal for:

  • Ground Level Ponds of All Sizes
  • Fish Ponds
  • Koi Carp Ponds

Drum Filters

OASE Drum Filters are highly sophisticated, with fully automated cleaning systems and waste discharge. Drum Filters are essentially flow-through filters in design, but these pond filters are built specifically for fine particle filtration, with the drum acting as a fine sieve, straining out all of the particle waste. Drum filters are primarily for larger ponds and more demanding filtration situations, high fish stocking and a lot of organic waste production in the pond, such as Koi Carp ponds that need to be kept clear. Drum filters are mechanical pond filters and should be coupled with biological filters to ensure that both solid waste and soluble waste are processed.

ProfiClear Premium Drum Filter

The OASE ProfiClear Premium L and XL Drum Filters are designed to be modular filtration systems, so that the biological Moving Bed Modules can be directly connected to the rear side of the Drum Filter, thus covering all bases. OASE also produce the ProfiClear Premium Compact filters, which combine the drum filter and biological filtration in the same filter housing for a complete single unit solution.

Drum Filter

Drum Filters are Ideal for:

  • Swimming Ponds
  • Koi Carp Ponds
  • New Build Ponds

What is the Best Pond Filter Media?

The best pond filter media depends on what the pond media is required to do. For example, if only one type of filter media is used to perform both mechanical and biological filtration, filter foam or sponge media would be the best filter media for this task, as this type of media has the physical properties required to intercept solid waste, and house colonies of bacteria to provide biological filtration too, although asking one type of media to cover all bases is not always the most efficient way for a filtration system to operate. If there is a mechanical prefilter such as a drum or screening mechanism incorporated into your filter system, particle bio-media made from plastic or ceramic is a better form of filter media, as a good particle bio-media is designed for maximum surface area, which means more of the important nitrifying bacteria can live on the filter media and process soluble waste most efficiently. Other specialist supplemental filter medias can be used to combat specific issues too, for example, if you have an issue with excess Phosphates, a Phosphate binding media would be the best pond filter media to use to reduce this.

These are some of the filter media types you may have come across:


Filter Foams

Filter sponges are the most common type of filter media found in pond filters and are usually incorporated in 2 or more grades of coarseness, with a mix of both fine and coarse filter foams installed within a pond filter. Sponges, otherwise known as filter foams, provide an abundance of surface area and act as mechanical filters as well as housing for biological filtration. Filter sponges do require regular cleaning to remain effective as filter media.


Filter Carbon

Carbon, or Activated Carbon / Charcoal is not a media you will find as a staple of pond filters, but is an effective supplemental filter media that can be used in conjunction with pond filters. Activated Carbon is known to be a water purifier that acts to collect and adsorb unnecessary elements and particles from the water. The attracting properties of carbon make it an effective water cleansing media, with the Carbon media acting like a magnet to small particles passing through the media.



Plastic filter media is one of the best filter medias that can be used for biological filtration, as it can be designed and produced specifically to incorporate maximum surface area on each piece of media. Surface area is key to biological filtration, as the more surface area you have in the filter media, the more aerobic bacteria can populate it and break down soluble waste. Some of the more effective plastic filter media types include K1, Bio Balls and OASE Hel-X 13, PondPads and ClearWave, all of which are incorporated into the OASE ProfiClear Premium Moving Bed Filters and Standing Beds.



GFO (Granular Ferric Oxide) is a supplemental filter media intended specifically to reduce the level of Phosphates in water, by acting as a binder as water is circulated through the media. This type of filter media rarely comes preinstalled in pond filters but is rather used as a reactionary solution to high Phosphate levels in the pond. Phosphate, although not directly harmful to aquatic life, is a major contributor to string algae growth (as a food source), so reducing Phosphate can be key to keeping algae growth down.


Mixed Media

Many pond filters will incorporate a mix of different filter media types, whether it be sponge and plastic for both mechanical and biological filtration, or multiple different types of biological filter media behind a mechanical screening filter. Some pond filters come with filter Zeolite cartridges to add a third component to the filtration process by absorption. As long as the fundamental requirements of filtration are accounted for in the filter media used, there are many acceptable combinations.

Should I Use a Clarifier Filter?

Yes, if you wish to prevent green water, a UV clarifier filter is essential for killing off single-celled algae which gives the pond water the appearance of being green. UV clarifiers kill off and bind microscopic algae cells into larger clumps of cells (flocculent mass), which are then large enough to be filtered out mechanically. Without a UV clarifier, individual living algae cells remain too small to be filtered out by any mechanical filter and remain alive and present in suspension, discolouring the pond water. Many pond filters, such as in pond filters, pressure filters and some flow through filters come pre-fitted with a UV clarifier. Other systems require a separate clarifier filter to be installed between the filter pump and the pond filter. Gravity fed filter systems often incorporate the UV clarifier filter at the rear of the filter system. As well as keeping the pond water clear, UV can also kill off waterborne pathogens and parasites, so incorporating a clarifier filter into your pond filter system has many advantages.

Ultraviolet Clarifier

Should I Get a Mechanical Pond Filter?

You should look to invest in a pond filter that is effective at mechanical processing. A good mechanical pond filter ensures that solid waste such as fish excrement and dead plant matter is removed from the water regularly, which prevents this from breaking down in the pond and creating an unhealthy living environment. Furthermore, if you do not have an efficient mechanical pond filter you often will not have efficient biological filtration either as solid waste entering the biofilter is detrimental to the aerobic bacteria living in the filter media. So a good mechanical pond filter is essential for maintaining good water quality overall.

Mechanical Pond Filter

Should I Use a Biological Filter?

Yes. A biological filter is just as important as a mechanical filter, as biological filtration is how soluble (non-solid) waste such as ammonia and nitrite are eliminated. In the absence of a biological filter to process the soluble waste, fish pond water becomes toxic and fish health is compromised. An effective biological filter relies on an active Nitrogen cycle, with enough aerobic bacteria present in the filter media to keep these toxic elements from reaching any significant levels. A supplemental pond aerator can vastly improve the efficiency of a biological filter too, as the bacteria enacting the biological filtration are oxygen dependent (aerobic) types, that thrive in dissolved oxygen-rich conditions. Essentially a complete pond filter system should be capable of delivering both mechanical and biological filtration to a high standard.

Hel-X 13 Biomedia

Best Filters for Large Ponds

For large ponds, only (non-pressurised) flow-through filters can accommodate the necessary water throughput to turn the pond volume over at an acceptable rate, so only a large flow-through filter system should be considered for a large pond. The OASE ranges of large pond filters are an excellent choice. The ProfiClear Premium Filter is regarded as the best pond filter UK wide, and the OASE BioTec ScreenMatic are some of the best large pond filters on the market today. These systems allow for high volume water circulation with an abundance of mechanical and biological filtration, combined with cleaning automation to reduce maintenance. If you are building a brand new large pond, a gravity fed filter system supplied via a bottom drain in the pond floor can be a highly effective way to filter the pond, and can often provide a neater installation than you would otherwise get with a pump fed flow through filter.

ProfiClear Premium Filter Set

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