How to filter  your garden pond?

How to filter your garden pond?

Jeremy, construction site supervisor, Cardiff

Guide written by:

Jeremy, construction site supervisor, Cardiff

131 guides

Filtering your garden pond's water is a way to ensure the water remains of good quality and has a correctly balanced ecosystem. It comes down to mechanical vs biological filtration, with the possibility of adding UV lamps. Starting with the submerged filter and through to the pressurized filter, here is all our best advice for obtaining a properly oxygenated and clear pond water. 

Important features

  • Mechanical filtration
  • Biological Filtration
  • UV lamp
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Why filter a garden pond


garden pond constitutes an entire ecosystem that, like any natural environment, is host to interactions between a multitude of living organisms. Whether it be plants, fish, insects or bacteria, these elements can quickly modify the quality of the water.

The water quality is measured by several factors, the most important of which are the amount of oxygen in the water and the amount of total suspended solids (TSS) of any kind (organic or mineral).

Heat, light and the simple presence of organisms in the pond water can contribute to the deterioration of the water quality. Signs of deterioration can include algae appearring, deposits forming at the bottom of the pond or the water becoming cloudy and tinged.

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The various garden pond filtration systems

The basic assumption is that any filtration system will manipulate the two following parameters:

  • The level of oxygen, which largerly depends on the organic matter in the water. The higher the level of oxygen is, the better the quality of the water will be. On the other hand, a pond with more living organisms implies a lower oxygen level;
  • The total suspended solids (TSS), which results from the organic or inorganic waste in the water. The lower the level of TSS, the clearer and more transparent the water will be.

Mechanical filtration systems

The term "mechanical" implies that the water is  pumped from the pond and routed, using  more or less pressure, through one or more gravity fed filters. The water can be re-inserted into the pond in the open air, to increase oxygenation, or otherwise. The most common mechanical filtration systems are submerged, pressure-fed or gravity-fed filters.

Submerged filters 

As their name suggests, these filters are installed directly at the bottom of the pond. They are generally low power as they are designed to filter the water of a small decorative pond, without plants or fish (maximum area of 5 m3).


Pressure-fed filters 

Placed outside the pond, these filters are designed to treat water in medium sized pools (up to 30 m3) that may or may not contain plants or fish. The more powerful range of systems can, in addition to filtering water, generate a fountain, water jet, waterfall  or even a small decorative stream.

Gravity-fed filters 

This is the elite range of mechanical filtration systems! Consisting of several tanks linked to one another, the water is transported due to exerted pressure. It is then transposed to successive tanks (equipped with different size filters, a UV lamp or filter sponge, among others) and is re-distributed into the pond without requiring any further pressure (in the open air via a waterfall, stream, etc.). They are ideal for ponds up to 150 m3. Gravity filters are especially recommended if you have fish in your pond. Both their filtration capacity and flow rate are generally high and they consistenly provide a good yield.


Biological filtration systems 

This is the most effective solution for treating pond water, no matter what the size. The basis of this technology is inspired by the approach used by biological wastewater treatment plants.

The idea is to for the water to be fed through a bacterial filter. When the water comes into contact with this bacterial colony, the bacteria attaches itself to it, degrading the organic matter and absorbing certain pollutants and unwanted compounds (such as nitrogenous materials).

For an even bigger yield, many biological filteration systems are composed of several chambers, each containing a different type of bacterial filter. The goal is to better treat pond water thanks to different bacterial filters of different types and sizes. This is also an ideal setup if you maintain a fish population in your pond.

Be aware that this type of filter requires seeding. In other words, to access the filter's maximum potential, you will need to wait for the bacteria to colonize the filters and attach itself to the different surfaces. If you hope to populate your pond with fish, wait until the biological filter is completely operational. It's best to start the filtration process several weeks in advance, depending on the volume of the pond, before introducing new fish. Some filtration systems, those designed for smaller tanks, can feature double filtration: mechanical and biological. They are an interesting alternative for small ponds with no fish.

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UV lamps: uses for filtration


UV filters (ultraviolet) are commonly used to destroy all living organisms in drinking water and they can often be referred to as sterilizers.

Some filtration systems on the market (most often mechanical) feature UV filtering on top of the conventional filter. The added technology will further controlthe bacterial population in the pond water and improve the treatment's efficiency. That being said, a UV lamp consumes electricity and its life-time is not unlimited. That's why it's important to make sure the UV lamp that you choose has sparts parts available!

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Factors to consider to correctly filter your pond's water


To choose amongst the different available filtration systems, here are a list of factors to consider, in order of priority.

The pond's volume 

The total water volume will dictate the required filtration capacity of the chosen system, the necessary pumping power, the size of the filters, and so on.

Fauna and  flora

Are you planning on having plants or fish? The more living organisms your garden pond has, the faster it will get polluted. To prevent this re-occuring pattern, you need to treat the pond water more frequently (impacting the pump's flow, the treatment volume, etc.).

Pond development

Are you planning on installing a waterfall, a fountain or a stream? Be sure to choose compatible system, one that offers that specific output.

Pond maintenance

This can include cleaning the filters, replacing the UV lamp or emptying the gravity-fed tanks. If given the choice, always go with the model that is easy to take apart and accessible with readily available spare parts.

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Guide written by:

Jeremy, construction site supervisor, Cardiff, 131 guides

Jeremy, construction site supervisor, Cardiff

Electrician by trade, I first worked in industrial estates where I installed, wired and fixed a large number of electrical installations. After this, I managed a team of electricians for this type of work. 10 years or so ago, I turned to building and construction. From the modest family home, to gyms and theatres; I have been able to coordinate, audit and organise all sorts of construction sites. for 4 years now, I am restaoring and bulding an extrension to a bungalow in the heart of the welsh countyside. My experience in manual work and my knowledge means I am proud to be of service. Terraces, interior design, roofing, plumbing, electricty, anything goes! We have, my wife, daughter and I, built almost everything we have from scratch! So to answer all of your questions, and to orientate and advise you on coosing your tools? Easy!

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