The basic types of biological filters for garden ponds and fish ponds: the first resort to getting your pond water clear.

The picture above demonstrates in a simple form the natural cycle of
organic substances (represented by the red arrows) being broken down in a pond with a biological
filter. This is called the Nitrogen cycle and in this case
demonstrates in particular what happens to fish food as it is digested
first by the fish and then the resulting faeces are then processed by
bacteria in the filter medium. The cartoon is one that featured in 'The Perfect Pond Detective Book 1: The biological balance' by Peter May, available from this site.

Biological filtration:

is a method of removing detritus/muck, organic compounds and specifically ammonia and nitrites from pond water by pumping pond water into a container and straining it through various foams, meshes or filtration media in which there is an established population of specific bacteria. These bacteria need plenty of oxygen to survive and they use it to digest organic compounds and a lot of the sediment that would otherwise lie decomposing in the same way on the bottom of the pond, although much more slowly. For small ponds there are two basic types of designs of filter that are built to work like this: Gravity filters and Pressure filters. 

Gravity Filters

A simple and very inefficient gravity filter in operation.A more sophisticated gravity filter with several chambers containing different media with an integral ultra violet clarifierGravity filters use a pond pump, usually submerged in the pond, to pump dirty water up to the filter from where gravity pulls it leisurely down through the filter media and it finds its way through to an outlet back into the pond. These types of filter have been around commercially since the beginning of the 1980s and so various types of media and methods of moving the water through the filter have risen in popularity and others have waned over the years. 'Add-ons' that have improved the effective removal of the algae or killed it or made it split apart or clump together have included the UVC (the ultra violet clarifier), magnets and air pumps. The jury is still out and internet forums still buzz with the the pros and cons of various filtration media and adding algicide mattings in which the media.

Some gravity filters push the water up through the media, others go sideways, others spin it round where a 'venturi effect' removes fine sediment. Bottom Drainage filtration systems are the most sophisticated filtration systems, which take the water from the bottom of the pond by underground drainage and feeds it to through a settlement chamber first and thence through a series of various types of filter medium to a chamber where is pump back as super clean water back into the pond. The secret of these is that the water level in filter chambers is at pondwater level and gravity effectively feeds the water  through the system because of the effect of the pump constantly reducing the water level in the last chamber. The advantage is that there is no pump in the pond and the pump in the filter is merely pumping clean water. It is an extremely quiet unobtrusive system, but it is big, perhaps a third of the surface area of the pond, and generally has to be hidden under some fancy decking. This refined style of filter will be covered separately in a later article. 

A sphisticated and highly efficient Cloverleaf filter uses a venturi effect to spin out the the sediment in the dirty water.

The design layout of a bottom drain style gravity fed filter showing the principles of operation

 Pressure Filters

An Oase pressure filter with a cutaway to demonstrate the flow of water and the remote sponge cleaning facilityPressure filters, sometimes refered to as 'Canister filters', work the same as Gravity filters but under the force of pressure from the submersible pump in the pond the container in which the filter foams sit is sealed and capable of being A diagram illustrating the way that the Oase Filtoclear filters work in a pondpressurized. This requires a sophisticated and reliable pump that is directly compatible with the filter and for which the specificatiions from the manufacturer match perfectly. To this end, it is probably best to buy this sort of filter as a kit or at least to buy the pump specifically recommend by the manufacturer. The reason the system is designed to work under presure is because that it seems that, per cubic centimetre of volume, these filters are more efficient at removing detritus from the water. They are not necessarily better at removing pollutants from the water by means of biological activity and they are also liable to clog up a lot quicker. This latter problem is overcome by either built-in automatic self-cleaning or a very easy maintenance by way of plungers and switches that are hardly any chore to operate. Small warning indicators are also used to declare how the filter is coping. Ultra violet clarifiers are often built in as an integral part of the unit thus increasing their effectiveness.

What is the best type of biological filter for you?

The first thing is to choose a good brand that you can rely on. There is no doubt that many manufactures make claims about the performance of their biological filtration units that are far beyond their performance in reality. Look around and try to get some feed back from users of various products. Having chosen one, try to get the product from a reliable supplier. A good filter and a good pump will carry a long guarantee, so you will want your supplier to be in existence for at least as long as that. So use your common sense. You will see if you have browsed in our shop that the only products we would whole heartedly recommend for the small pond owner are the Oase ranges. This is quite simply because the Oase products have been consistent in their quality ever since they emerged on the UK market back in the early 1980s. They have always outstripped their rivals in longevity, design and performance. They are perhaps the most expensive products to buy on the aquatics market, but if you work out their economy of operation and work in the life expectancy of the pumps and other kit, you will find out that the pound for pound and penny for penny you save in the long run. Add to that the peace of mind and the ease of maintenance built into their design, then you are into the bonus zone. You will find the gravity an pressure filter and pump kits here. The individual gravity and pressure filters are here when they are available.

Considering gravity or pressure filtration: pros and cons 

• 'Gravity filters' needs less maintenance. They also have less limitations on their capacity.
• The flow through the gravity filter must be matched to roughly half the volume of the pond every hour. A pump that is capable of producing too much water for the filter can easily be 'tee'd off'.
• Gravity filter maintenance is easy if there is a 'backwash facility' with the filterand that some where to flush filthy waste has been built into the water garden infrastructure. This is not always the case if the filter has been added as an after thought, in which case the muck as to be scooped out and the filter medium cleaned by hand.
• 'Pressurized' is less obtrusive in a small garden and can be placed or dug in anywhere. They are more suitable for smaller ponds.
• They need a fairly forceful pump to make them effective and if you are using the outlet for a source of waterfor a waterfall, you will find the performance dramatically reduced. it is best to use the pump that the manufacturer specifies. They cannot be 'tee'd off' since that will reduce its effective working pressure. In order to get more flow to a waterfall, you will need another pump.
• Many people when flushing out the waste from a pressure filter just spray it onto the garden.
• Despite needing regular maintenance, it is quick and easy to do with pressure filter for most of the year.