Biological Filtration

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There are a few different types of filtration that apply to aquarium filters; biological filtration is the most important as far as the fish's health is concerned.

The principle behind biological filtration is to supply a large surface area on various media(s) and pass oxygenated water through the media. This high surface area / high oxygen content environment is ideal for the types of bacteria which process waste, to colonise and spread.

The bacteria involved primarily convert ammonia compounds (waste product) into nitrites, and then further bacteria convert the nitrites into nitrates. The resulting nitrates should be kept at low levels through a combination of water changes, use by plants, chemical filtration, or by anaerobic bacteria elsewhere in the aquarium. The cycle of events that completely remove waste products is called the nitrogen cycle and depends heavily on biological filtration.

In many smaller, or internal, filters, the surface area for bacteria is provided by specially designed sponges which double as mechanical filters by trapping visible particles.

In larger, or external, filters, specially designed media is used which has a very high surface area when looked at under a microscope. This 'biological media' allows more bacteria, and hence, better biological filtration.

The bacteria that carry out biological filtration can only spread in response to the waste being produced in the aquarium, as this is their food source. This is why aquariums must be stocked slowly, in order for the bacteria to adapt to the increase in waste.

A filter that carries an established population of bacteria is the result of many weeks, or months of growth and colonisation in response to the aquariums waste levels. If those bacteria were destroyed, levels of toxic ammonia would quickly rise and result in disease and death to the fish.

Bacteria involved in biological filtration are most commonly significantly reduced by the following common factors:

  • Washing biological media in tapwater (Chlorine kills the bacteria)
  • Extended power cuts (Oxygen loss inside filter kills bacteria)
  • Complete replacement of media
  • Environmental shock e.g. sudden temperature change

Although Nitrosomonas and Nitrobacter sp. are often stated as the bacteria species that carry out biological filtration, it is actually a number of different species of bacteria that are responsible. Because the species involved are not exactly known, and the correct colonisation of a filter depends on each aquariums variable conditions, it is wise to treat filter bacterial additives with a pinch of salt. These additives are designed to 'kick-start' the colonisation of bacteria, in order to process waste and mature the aquarium faster, so that fish can be added and waste products removed quicker. In many cases however, these additives simply introduce waste products, and bacteria then grow to remove them. This is fine if there are no fish present, but whilst fish are present, the introduction of these wastes can be very harmful, as it will take the bacteria time to remove them. This is one reason amongst many why water testing is an important part of fishkeeping.


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