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Filters & Their Maintenance

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A guide to the different types of filters available and how to care for them. 

Filters & Their Maintenance


Anyone who is keeping fish should have a filtration system set up in their pond. This will not only ensure that the water is kept clear, but will also cause any toxins to be removed from the water before they cause any harm to the inhabitants of the pond.

Regardless of the type of filter you have, all follow the same stages:

Ultra-Violet Clarifier (UVC)

The UVC subjects the pond water to UV radiation.  This means that microscopic single-cell algae flocculate (stick together).  The resultant large particles are picked up by filter foams and prevented from re-entering the pond.  This prevents your pond looking green (like pea soup).

Mechanical Filtration

Pond debris such as fish waste and dead pond plant particles are caught in mechanical barriers -typically foam or brushes.  These foams are often graduated to catch even the smallest pieces.

Biological Filtration

Finally, pond water will often pass through a bed of bio-media. This bio-media is often unusually shaped, with lots of ridges and undulations, designed to maximise surface area which allows friendly bacteria grow on them.  These bacteria will break down ammonia, which is caused by dead and decaying organic matter, into nitrite.  A different type of bacteria then breaks this dangerous nitrite down to nitrate and will be absorbed by plants as they grow.


There are two categories that filters will generally fall into, pressurised and box. Both have pros and cons which we will look at below.


Water flows through these filters very quickly as they are sealed units and thus maintain the pumps’ pressure. This means that they do not provide a great level of filtration as the water spends less time inside them and there is not chance for all the dirt to sediment out (sink to the bottom).

The major benefit of these filters is that they can be sited away from the edge of the pond, buried or hidden amongst shrubbery. They can also be installed below the level of the pond as the water can be ‘pushed’ out of the filter and up to a waterfall, over the edge of a pond or up a bank.

Most modern filters, including our Spinclean range, also have a backflush function to assist with cleaning the filter out. This reduces the frequency of manually cleaning the filter out, especially when compared to traditional flow-through filters. For this reason, these filters are ideal for those wanting to reduce the amount of time spent cleaning out their filter.



Water flows through these filters slowly as they are not sealed units.  This means that they provide a better level of filtration than pressurised filters as the dirt has more time to pass through the mechanical stage and more sediment is filtered out. This makes box types units better options for Koi ponds, as they typically produce more waste.

The drawback of these filters is that they cannot be sited away from the edge of the pond. This is because they have no pressure to ‘push’ the water anywhere and so need to be at the edge to allow the water to fall back into the pond naturally.

For smaller ponds, these units can also offer a more cost-effective option as some units are offered without a UV. 


These units are complex versions of the box filters (with some extra special features) which are designed to cope with the extra dirt that the koi produce.

The drawback of these units tends to be the price, as well as the same positional problem as the box units. However if you want to keep a large stock of koi and would like the water to be as healthy and clear as possible then these are the ideal choice.

For more advice on choosing the right filter for your pond, please consult another of our guides on How To Choose A Pump & Filter


Filter Maintenance


Irrespective of what type of filter you have, you will need to maintain it regularly to ensure maximum filtration of your pond. Some filters are fitted with a ‘quick-clean’ or ‘back-flush’ function, these are very helpful for weekly maintenance but should not be relied on wholly. You will still need to disassemble your filter regularly to clean out the foams thoroughly.

It is vital that you rinse any foams, brushes or other filter media in a bucket of pond water and NOT tap water. Tap water contains chlorine and chloramine which will kill all the good bacteria that develops on the media. These good bacteria are essential to maintain a healthy balance in the pond and will assist in keeping green water at bay. Bacterial treatments are widely available to help boost the performance of the filter, such as PondXpert Gel Balls or our latest treatment the Pondxpert Filter Start Gel


UVC Maintenance


IMPORTANT! Always disconnect the UVC from the power supply before carrying out any maintenance work and never look directly at an illuminated UV bulb.


We recommend replacing the UV bulb after 6 months of continuous use, for most pond keepers this will mean replacing the bulb once a year. Although the bulb may still be shining, after around 5000 hours it loses all effectiveness and will not assist in clearing the pond of green water. It is advised that you replace any O-rings or seals at the same time as changing the bulb to ensure it is fully protected.

UV bulbs are surrounded by a quartz sleeve, this is a glass tube that protects the bulb from the water passing by it. Quartz sleeves are very fragile and should be handled with care. Limescale and dirt can build up on these quartz sleeves and prevent the UV rays from treating the water effectively so will need cleaning every few months.

***Always consult the manufacturers guide for specific instructions on how to replace the bulb***

Originally published 19/01/2018, Updated 16/11/2020






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