As water is the fish's natural environment, it follows that its quality should be maintained at the highest level at all times. This is best achieved by using filtration equipment in association with regular partial water changes.

Water can be cleaned by filtration in three methods -

Mechanical, Chemical and Biological.


Mechanical Filtration removes of solids and physical impurities from the pond or aquarium water by means of sedimentation, settling, or the use of some method of physical straining through a sponge-type medium (in aquariums) or brushes (in ponds). This removal of the visible water pollution will not on its own necessarily provide the best environmental condition for fish.


A material such as activated carbon will adsorb pollutants from the water, removing any yellow colouration in the process. Other reagents may be used to change the water's properties: for instance, zeeolite can be used to reduce hardness.
Zeolites, Activated Charcoal, pH Buffers and other commercially available chemicals all have a use in practical fishkeeping but you are advised to 'complete the basics' prior to adding these sophistications to your filter circuit.

Filter media such as sponges, activated carbon, zeeolite etc can easily be contained in chambers within the filter unit itself.


Biological Filtration is not, in fact, filtration but purification and makes use of naturally-occurring bacteria to remove toxic waste products (generally ammonia-based) and other chemical impurities from pond or aquarium water.

The general principle is to encourage bacteria to colonise as large a surface area as possible, usually within the gravel itself (undergravel filtration) or in a special filter unit containing fine silica sand (fluidised bed filters).
A constant water flow through these two types of media is vital to mantain bacterial life and care should be taken not to interrupt any power supply to these types of filter.

Most filtration media have one main function (Mechanical or Biological) but there is usually a degree of both in any filter once it matures. A biological system may take several weeks before it becomes mature, and should never be overloaded by adding too many fish at any one time.

Always clean filter media in aquarium or pond water;
using tap water will kill off any beneficial bacteria.

Where a filter has several layers of media, it is good practice to change only a part of this media at any one time in order to maintain the filter's bacterial efficiency.


  Internal filter          External filter

Aquarium filters, now generally operated by an electrically-powered impeller rather than by air from an air-pump, can be for either internal (submerged in the aquarium water) or external (sited outside the aquarium and connected to it by hoses) use.

The latter method is slightly more easy to maintain and disturbs the aquarium less when servicing is required.

Most filters are 'dedicated' to certain standard aquarium sizes, so make sure you get the correct size for your aquarium.

Pond filters are usually hidden in the rockery beside the pond and can incorporate not just sedimentation and biological filtration chambers but also ultra-violet lamps too.

Large Koi pond filters (usually at least one-third the size of the pond) are fitted in the ground and connected to the pond by drainage pipes installed during the pond's initial construction; they can be 'gravity-fed' with a pumped return to the main pond after cleansing has occurred.
This vortex filter can be just a part of a Koi pond's comprehensive filtration system!

It is good pond management practice to fit all external filters with 'back-flushing' facilities in order to periodically remove accumulated sludge from the filter.

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© FBAS 1998 RCM/RDE           Aquarium Management Care Sheet 1 1/2

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Last updated July, 2005