Buying the right pond filter


Pondering some fish outdoors? Danny Crook of OASE offers some pointers to make sure you get the right set-up.

Pond filters over the decades have often been a bit hit and miss and many were DIY affairs; functional, but not always efficient. Huge, high maintenance designs that held vast quantities of different medias have thankfully been replaced over the last 20 years or so with efficient, easy to clean filters that have been scientifically researched and developed. Greater understanding of how filters and their bacterial inhabitants work has enabled pond owning to become easier than ever.

In 1996, OASE launched the very first complete filter set including a combined pump, filter and UV on the European market. Until that point, filtering a pond involved buying equipment separately, often from different manufacturers, and fitting it together with hosing and jubilee clips. 

Putting the right components together worked OK, but it wasn’t an exact science and was rarely neat or maintenance friendly. Nowadays it is easy to buy a complete set, perfectly researched and packaged for your size of pond, made up of component parts from the same supplier that complement each other and fit together perfectly and easily.

Mechanical and biological

To know how a filter system is developed, you need to understand the three main principals of filtration, how they work together, the importance of each step and the best conditions to provide the best results. 

There are two main jobs going on in every filter. The first is mechanical filtration, the removal of solid waste and debris from the system. The second is biological filtration, the breakdown of toxins like ammonia and nitrite by encouraging the growth of beneficial bacteria.Mechanical filtration can be performed using anything that will trap particulates and remove them from the pond water. Settlement chambers can be a mechanical filter (although they take up a lot of space), but sponges and brushes have historically been the most widely used. In larger filters these have been increasingly replaced with sieves and screens which can be cleaned more easily whilst also removing wastes of a much finer particle size. It’s important to note that the mechanical stage performs two fundamental tasks; removing particles so the pond looks cleaner and helping keep the biological stage cleaner.

With biological filtration, it’s vital to know that bacteria like to establish on a clean surface. You may have the best biological media on the market, but if it clogs up with waste then it is no better than gravel or sponges. 

At this stage, it is important to briefly mention biofilm, the substance that bacteria produce to hold on to a surface. Biofilm is a yellowy brown colour, so healthy, well colonised biological media will discolour. Avoid confusing this with particulate waste which is darker brown and can impact on the space available to the bacteria. One discolouration needs encouraging and one needs cleaning.   

An efficient and mature filter will produce nitrates as an end product, a favourable algae food. Many pond plants are excellent nitrate fixers, but in Koi ponds with little or no plant life then the nitrates will inevitably feed algae. 

A third type of filtration is rarely used in pond filters — chemical filtration. Using carbon, zeolite, and ion exchange resins is common in aquaria but can also be used to great effect in ponds. Chemical filtration is popular across Europe but has somehow passed us by in the UK, but there are medias available that remove phosphates, that absorb ammonia, and medias such as carbon that can remove tannins and staining. Essentially, any chemical media will need replacing or recharging regularly. This is vitally important — chemical medias can ‘dump’ their absorbed loads back into the water with disastrous results if not maintained.


A pond filter is often a case of ‘out of sight, out of mind’ and maintenance ignored until the pond starts to look dirty. Any pond filter needs looking after to get the very best results, and even ‘self-cleaning’ filters are not fully self-maintaining. 

Check any pond filter regularly. Lift the lid off of it and have a look inside to see if it’s dirty and clogged. With pressurised filters or pond filters where it is not so easy to look inside, you can instead pay attention to flow rates — a blocked filter will experience reduced flow, and needs a clean. 

Look upon maintenance time as improving your pond and the wellbeing of the inhabitants rather than it just being a chore — little and often is always a great motto to live by with all aspects of fishkeeping. 

Remember that biological media should only ever be washed in water from the pond. Don’t use the garden hose pipe as the chlorine and chloramines in the tap water can kill off your beneficial bacteria.

Replacing media 

Occasionally, media may need replacing, for example if sponges fail to regain their shape after cleaning. Damaged screens or sieves should always be replaced. If brushes are losing their bristles then they should also be replaced. 

If the media is mechanical then it can be replaced straight away, however biological media should be replaced in stages. Remove no more than a third of the old media and replace with new. A week or so later replace another third and again a week or so after that.

UV filtering

The use of ultraviolet (UV) light is now commonplace. A UV is an essential part of a filter system and can enable manufacturers to offer a clearwater guarantee, but it cannot work alone. It is only the right combination of pump, filter and UV all working together that can guarantee to keep a pond clear. 

Green water is the accumulation of single celled algae that are too small to be trapped by mechanical media. When algae and water cells are passed through an ultra violet light unit, the UV corrupts them and the walls of the algae cells start to break down. This makes them sticky and causes them to clump together (flocculate). These flocculated clumps of cells are then large enough to be trapped by the mechanical part of your filter and thus your pond stays nice and clear. 

Without a filter, the flocculated algae settles in the pond and dies off. The dead flocculated algae then disperses and you end up with a brown and murky pond rather than a green and murky pond!

What’s available?

There are the pros and cons to each type of filter available, and it’s inevitable that some are better suited to the particular needs and size of your pond. Once you have decided on which type is best for you then you can start to work out which model is right for your pond. In the descriptions below, features or benefits that are either unique to OASE or not available from all makes on the market are marked with a *.

IN-POND FILTERS: These filters are compact all-in-one packages with pump, filter and UV in a box as small as 20x23x13 cm. All equipment goes in the pond with nothing outside. 

 +  Everything in one package, pump, filter and UV

 +   Only one power cable*

 +  In pond; no visible equipment outside

 +   Compact unit

 +   Clearwater guarantee available*

 -  Only suited to small ponds

 -  Maintenance. You have get it all out of the pond to maintain it

PRESSURISED FILTERS: These filters sit outside of the pond and are supplied by a pump situated inside the pond. As the lid of the filter is sealed tightly shut, the water can travel from the filter ‘uphill’ to a waterfall and the filter can be partially buried to help conceal it. Many filters have easy clean solutions that allow for regular maintenance without even having to remove the lid. 

 +  Sealed unit, ideal for easy concealing in the garden

 +  Integrated UV light*

 +  Easy cleaning solution*

 +  Clearwater guarantee available*

 +  Can be purchased as sets with perfectly sized pump*

 -   Compact size means more regular maintenance. It’s easy to do, but it will need doing more regularly than other filter types.

 -   Limited sizes available

 -   Not suitable for very large ponds

BOX FILTERS: Also called ‘flow through’ filters, once upon a time every filter was a box filter. Unlike pressurised filters, these do not have a sealed lid and can’t send water ‘uphill’, so positioning needs some thought. The box filter can be split into two further categories.

 +  Larger filter units require less regular maintenance

 +  Integrated UV light*

 +  Easy Cleaning solutions*

 +  Clearwater guarantee available*

Can be purchased as sets with perfectly sized pump* Huge range of sizes available

 -  Can be more difficult to conceal in a garden as pump fed systems need to be above ground. 

 -  Gravity fed systems initially more expensive to install.

 -   Gravity fed systems require bottom drains in the pond or similar


PUMP FED SYSTEMS: Fed from a pump installed in the pond, these are sometimes referred to as ‘gravity return’ filters. The water runs through the filter, passing through various media, and then clean water ‘falls’ back into the pond, meaning that the outlet has to be at the highest point of any pond system. If you want the water to run down a waterfall then the outlet of the filter must be above the top of the waterfall. Some modern designs have mechanisms that enable you to clean the filter without getting wet or dirty. Pump fed systems can vary from small 28x38cm boxes to multi-chamber systems that can be connected over 4-5m. 

GRAVITY FED SYSTEMS: Often seen as the reserve of the serious Koi pond, a gravity fed system needs to be built into the pond as it is constructed. Water enters the filter under the pressure of gravity (usually via bottom drains) and is pumped back into the pond. Gravity fed filters are installed
in the ground at the same water level as the pond and are often hidden beneath decked areas or in special filter housings. Although they can be expensive to install, the lack of equipment inside the pond gives a clean and professional look. 

PUMPS: All of the waste and algae that you want to remove is produced in the pond. To remove it you should use a proper filtration or solids-handling pump to get even large particles out of the pond and into the filter. A properly designed filter pump will produce a good flow rate and move solid particles from 6-11mm in size.
A fountain pump with a fine strainer will block more regularly and won’t take the waste out of the pond. 

FLOW RATES: When looking for a pump, make sure that the water volume of your pond is turned over at least once every two hours. For example, a 1000 litre pond needs a turnover of at least 500 lph. 

Why this flow rate? Well, the algae cells that cause green water can divide into two every two hours on a sunny day, so they can double your algae burden every 120 minutes! Turning over a minimum of once every two hours ensures that the algae passes through the UV quicker than it can multiply. 

Don’t trust the number on the box!

Filters are often allocated model numbers for the volume of water that they can look after without fish present. This sounds odd to us in the UK, but many ponds across Europe are just garden ponds with no fish in them. The UK is often referred to as a nation of animal lovers and we generally build fish ponds. 

A garden pond with no fish, such as a natural or wildlife pond will be easier for a filter to look after — add fish to the mix and the filter has to work harder. Chuck a few Koi in, grubbing around and uprooting plants, and the filter gets pushed to its limits. 

As a general rule of thumb, whatever the model number of a filter, halve it to get a realistic idea of its fish pond capacity, and halve it again for Koi ponds. So, a filter rated to 20,000 litres will look after 20,000 litres with no fish in the pond, 10,000 litres with goldfish in the pond and 5,000 litres with Koi. 

Biologically the 20,000 filter will cope with even a 20,000 litre Koi pond, but only if the media is kept clean, and mechanically the filter would need considerable maintenance. Use the same filter on a 5,000-litre koi pond and with the right stock and feeding, maintenance should be a much more manageable every 3-4 weeks. The sizes are recommended to hopefully give you the water gardener a much better pond experience.