Mechanical Filtration

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 (media) to achieve clarity to enable you to see your fish. This removal of, say, the visible water pollution will not on its own necessarily provide the best environmental condition for fish.

Chemical Filtration

This involves the use of reagents such as activated carbon to adsorb pollutants from the water on one hand, or to change the properties (hardnes/softness, acidity/alkalinity etc) of the water by using the relevant materials. For instance, activated carbon will remove yellow colouration from the water, and 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.

Biological Filtration

Biological Filtration (undergravel filtration, fluidised bed filters) 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, thus safeguard the health and well-being of your fish.

'CLEAR water is not always HEAL THY water!'

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.

Protein Skimming

Of particular importance in marine aquariums, this is another method of removing organic material from the water. This is achieved by using fiercely aerated water within the filtration unit: organic matter is attracted to the water/air bubble interfaces and the resultant pollutant-laden foam is collected in a chamber from where it can be regularly emptied.

Ultra Violet Units and Sterilizers

Ultra Violet Units, used correctly are sterilisers and are capable of 99.9% sterilisation of water. Used in conjunction with a filtration system, they will indirectly clear algal growth (Green Water) in pond or aquarium and have a place in practical fishkeeping if used correctly. UV Light can be dangerous and such units should be used with care and discretion.

Similar, but not so generally known, are Ozone generators. These utilise high voltage discharges to generate ozone which, unless properly controlled and dissipated, can be just as effective in removing your fish as well as all the potential nasties at which you aim it!

These are the basics, the sophisticated hardware now available is merely utilized to achieve the same objectives via another route.

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