Choosing the right pond filter

If you drive a car, the odds are you will fall into one of two categories. Those whose priorities are the looks or a car and the driving experience, and those who are interested in what’s happening under the bonnet. I fall into the first group, and struggle to remember the last time I lifted the bonnet. I think garden pond filtration has many similarities.

Admirers of a beautiful pond will be captivated by the tranquillity of cascading water and shoals of colourful fish gliding through the depths. They have little interest in what is going on behind the scenes to create such an impressive pond and their impressions of such a perfect and serene world would be dashed if the bonnet were to be lifted and they were shown the engine (ugly pipe work and filter units) 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.

1. An engine is critical to the performance of a car, but does every pond need a filter?

When a pond filter is optional.

If your objective when installing a pond is to recreate a slice of nature and to attract wildlife to your garden, then by design, your pond will not require a filter. Such wildlife ponds by definition are not heavily stocked with fish and can adequately rely on dense planting to create and maintain the pond’s balance. A pond filter is also optional should you want to make your pond a half-way house between a stillwater wildlife pond and a heavily stocked ornamental fish pond, with a pump installed merely to add a little moving water ‘on demand’.

2. Do I need a filter?

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.

2. What are the benefits of filtering a pond?

The majority of new garden ponds are filtered because of the many advantages filtering offers both the pondkeeper and their fish.

A. Fish. A filter performs several complimentary roles that help maintain the pond in a suitable condition for fish. Besides obviously removing solid particulate debris from the pond (such as material from fish, food and your garden) a pond filter eventually matures to become a supportive environment for beneficial bacteria that breakdown largely soluble waste that would otherwise accumulate to toxic levels.

B. Water. Circulating water is likely to be oxygenated water, which is then redistributed throughout the entire pond. A filter also provides us with the opportunity to install an Ultraviolet clarifier (UVc) in line between a pump and the filter. The UVc is now a guaranteed method (by most manufacturers) of creating a pond with crystal clear water. A UVc causes the microscopic algal cells that cause green water to clump together, but unless there is a filter installed to remove these clumps of dead and dying algae, they will simply recirculate around the pond, making your pond look like a snow storm (but with green snow!). So by installing a filter with a UVc (or a good selection of plants) a filter will give you a crystal clear pond.

C. Peace of Mind. A filter does a lot of the leg work that is performed by the host of organisms that help to purify the water in a naturally-stocked water body. A biofilter gives a pond far more capacity for holding fish compared to an unfiltered pond which in practical terms means you can stock your pond with more fish, without the worries you would have if you had no filter. A filter also allows you to feed your fish more intensively should you wish, with a reduced risk of polluting your pond.

D. Reduced Pond Maintenance. A submersible pump that is placed on the pond bottom will continually dump solid matter into the filter, keeping the pond relatively clean and sediment-free. Regular maintenance of the filter will mean that the days of cleaning out a silted-up pond will be put off for years.

3. What do I need to know before I can buy the right filter for my pond?

A. Size of pond. Filters are generally rated to the volume of a pond (which in turn is rated to the number of fish a pond that size could hold). As the majority of ponds are smaller than 1500 gallons, most filters are manufactured to suit ponds at approximately 500 gallon steps – Those under 500 gallons, 1000 gallons and 1500 gallons.

B. The number and type of fish to be stocked. The filtration needs of a garden pond that is lightly stocked with a mix of pondfish will be far smaller than if that same pond was densely stocked with ravenous and rapidly growing koi. Filters for koi ponds are more substantial in size and design, usually being divided into a series of different chambers. Whereas, a garden pond filter will be smaller (and less expensive) and generally consist of a single chamber.

4. What options are there before I choose a filter?

1. Internal Filters. Internal filters sit inside the pond and are only really suitable for smaller ponds. They should not be confused with the strainers that are placed on a pump’s intake to protect the impellor from taking up debris. Internal pond filters generally consist of large foam blocks that replace the pump’s strainer and benefit from being concealed within the pond. They are also easy to fit, but can have a tendency to block up easily, reducing the output of the pump. Cleaning them can also disturb life within the pond.

2. External Filters. These consist of an external chamber (or chambers) that are placed outside of a pond. They are either pump-fed (see later) or gravity-fed where the filter is buried and runs at the same level as the pond. The water from a gravity-fed filter is returned to the pond from the final chamber via a pump whereas the water returning from a pump-fed filter, depending on the design, will either return under gravity or under pressure from the pump.

The pump-fed single chamber filter units (either trickle or pressurised) are the most widely used filters. They can be installed in a matter of minutes (even on an existing pond) and because the largest units can filter a 1500 gallon pond, it is easy to find an off-the-shelf filter that is up to the job of filtering most ponds.

The guts of a filter are made up of a combination of inert, washable filter media – usually a porous bio foam (in several grades) used conjunction with ceramic media of chopped-up pipework. The media will take months to fully mature and should be rinsed carefully to prevent it from blocking up while trying not to disrupt the beneficial bacteria.

Pressurised of Trickle?

One of the obstacles when installing a pump-fed trickle filter is how to conceal it – often resorting to hiding it behind rockwork, feeding a waterfall. A pressurised filter solves this problem by being able to bury the unit underground, with the return simply consisting of a hose feeding either directly into the pond of via a waterfall. Pressurised filters can have a tendency to collect more debris over a shorter period of time, making it necessary to carry out more regular maintenance.

Gravity-fed filtration. This represents the premier type of filtration and is tailored to ponds containing several thousands of gallons, usually heavily stocked with koi. Gravity-fed filters need to be planned as an integral part of the pond’s design and will consist of a series of chambers, divided into mechanical and biological functions. These units alone can cost more than an average garden pond and can be bought ‘off-the-shelf’ or constructed in-situ with the pond out of concrete blocks and fibreglass; – Something for the serious koi keeper with a handsome budget to spend.

What are the running costs?

If a filter does not have any running costs, how does it work?

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.

6. What about maintenance?

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.