Garden pond filtration

It is a long time since I last watched The Wizard of Oz (I didn’t catch it this Christmas) and I can’t recall the finer details of the ending. But what I do remember is towards its conclusion, experiencing a real anti-climax when we were shown the wizard’s true identity. Instead of marvelling at the amazing actions of a supernatural figure, we see an old man, tucked behind a curtain busily pulling levers and pulleys.

Garden pond filtration is rather similar. Admirers of a beautiful pond are captivated by the tranquillity of cascading water and shoals of colourful fish gliding through the depths. Their impressions of such a perfect and serene world would be dashed if they were taken behind the curtain to witness the levers and pulleys (unglamorous pipe work and filter matting) that are used to achieve the result. That is essentially the role of a pond filter. The ‘back room boy’ and unsung hero of a thriving pond.

Why filter?

Two reasons for filtering a garden pond include:

1. To achieve a healthy water quality to maintain fish health.

2. To provide clear water in which fish can easily be viewed.

These two objectives should be taken as a whole as they can both be achieved through the single process of filtration.

Pond fish are just like any other animal in that they excrete waste which would be toxic if allowed to accumulate in their body. We have long recognised the link between poor sanitation and disease and have invented practical water treatment solutions to reduce the risks to human health. Fish experience exactly the same threats to a healthy life if they are also exposed to a build up of toxic waste and such risks can be reduced by installing a pond filter.

Natural vs Artificial.

In natural balanced water bodies, such as oceans, rivers or lakes, fish are in balance with their environment. They are so lightly socked in relation to the water volume that there is no build up of fish waste. Their natural aquatic environment is self- sustaining. This is not true in most garden ponds that are typically well stocked with fish in all varieties and sizes, well above the stocking levels that would be found naturally.

Fortunately, an effective garden pond filter can be bought, complete and ready to go, with many units fitting in the boot of a car. Adequate filtration cannot be achieved by the small foam pre-filter placed on the intake of a pump.

How does a pond filter work?

A garden pond filter’s function can be broken down into 2 different yet complimentary functions:

1. Solids removal

2. Biological Filtration

Function 1- Solids Removal

A filter’s first function is to remove solid matter that is pumped from the pond. This could vary in size from fallen leaf matter down to microscopic particles that may make the water slightly cloudy. Solids are the first to be removed first to enhance the subsequent biological filtration processes.

Traditionally, mechanical filtration is likely to be the most limiting part of a pond filter. Most filter space should be designated for solids removal as debris will soon collect (especially if a UVc is installed) and then pass through to the other filter media.

Entrapment is the method used in standard external black box biofilters, where 2-3 grades of foam, running from coarse through to fine act to trap solids as they pass. Acting in a similar way to a sieve, the first filter media that the pumped dirty water encounters is quite coarse in structure, trapping and removing suspended solids from the water.

Many submersible pumps are supplied with a foam or perforated plastic guard to prevent debris from choking the impellor. This can prevent leaves and other larger solid particles from reaching the filter, being retained in the pond, causing the water to cloud or silt to build up on the pond bottom.

Clear isn’t always healthy.

Just because water is clear doesn’t mean that it is healthy. Solids are removed largely for aesthetic reasons, as pond fish quite paradoxically prefer the turbid waters of a clay pond. Ammonia and nitrite are soluble, colourless and undetectable to the eye and these are toxic to fish. They are broken down into less toxic substances through biological filtration.

Function 2 – Bio-filtration

Having safely removed any solids from the pond water, the clear water now passes through the part of the filter specifically designed for bio-filtration. As its name describes, a bio-filter is a living filter, colonised by many millions of bacteria whose role is to consume and breakdown the toxic ammonia that is constantly being excreted by fish (and other aquatic organisms).

These beneficial bacteria will colonise any hard surfaces (including the pond liner, pipework and rockwork making them feel slippery and slimy). However, a bio-filter is designed to provide a vast surface area on which these bacteria can colonise, providing the surface area in a filter which may naturally be found in many square metres of a natural pond or lake bottom.

Keeping a filter alive.

These well-housed bacteria are provided with a luxury lifestyle, receiving all their requirements for a long and healthy life. The steady turnover of water through the filter provides a constant source of ‘food’ – in the form of ammonia, as well as an essential supply of dissolved oxygen. It is recommended that the pond volume is turned over at least once every 2 hours.

As this vital part of filtration is ‘living’, unlike mechanical filtration, the bacterial colony takes time to become established or ‘mature’ and a filter must be run-in gently over the first months of its life. Fish should be added a few at a time, so that the bacteria can adjust and catch up with the rate of ammonia being produced by the fish. If too many fish are added too quickly, then ammonia levels will rise rapidly, causing fish stress and ultimately leading to disease.

Watch out for nitrite as well as ammonia!

Aerobic (oxygen-loving) bacteria breakdown the toxic ammonia into nitrite, which unfortunately, is still toxic. In fact, nitrite has a nasty habit of being more difficult to break down than ammonia and will persist longer than ammonia in water that is suffering a quality problem.

Care of the filter

Filters must be treated like a living entity. If they are not provided with oxygen, water and food, they will deteriorate and die. For this reason, a bio-filter must be run continuously, ensuring that the bacteria are provided with the materials for life.

There are times, as with any filter, that it must be cleaned and maintained. In the summer especially, waste will build up rapidly within foams, and these should be cleaned out before they clog or restrict the filter. This can be done without disturbing the more sensitive bio-filter.

In a box filter, where the foam layers may act as both mechanical and biological media, care must be taken when rinsing out the foams.

Bacteria are very sensitive to changes in their environment and any adverse action could set the filter’s maturity and efficiency back months. For this reason, when rinsing out the foams or cleaning any biological media, buckets of pond water should be used. If raw tap water is used, then chlorine and other variations in the water quality can have a detrimental effect on the bacteria.

Kill blanketweed and string algae.