Why is filtration necessary?

Nature sets the right example

Every aquarium is a microcosm in its own right. As simple as it may appear at first glance, an aquarium is actually subject to the same complex laws of nature as prevail in such a vast body of water as the Amazon.
An understanding of the biological processes involved is crucial for a successfully functioning aquarium. The fish will then remain healthy, the plants will flourish, the water will be brilliantly clear and troublesome algae will remain in the background. Filtration plays a key role here. In nature, water undergoes continuous slow filtration and is continually replenished. It seeps through different strata of the earth and rocks. Gravel and sand in the beds of natural bodies of water are colonised by countless mircoorganisms and act like giant filters. Fish excrement is biodegraded here by bacteria. Microorganisms break down and remineralise dead plant matter. Suspended matter settles at points where the water flows more gently. The water which trickles or flows away is replaced by rainwater.

The aquarium filter

The filter is one of the most important components for a successful aquarium. Apart from its mechanical clarification functions, it serves above all to condition and "detoxify" the water by biological means. Incorrect filtration is a common cause of ailing fish, poor aquarium plant growth and algae infestations.
The biodegradation of contaminants thus constitutes the core function of every aquarium filter. The waste substances - invisible to the naked eye - which can become harmful to fish are broken down here. This work is carried out by filtering bacteria. Each strain of bacteria has its own special "preferences". Some degrade toxic nitrite, for example, while others feed on organic suspended matter or break down dead plant matter.

The main filtering material is the decisive factor

The effectiveness of a filter is ultimately dependent on the employed main filtering material. This material offers the filtering bacteria the necessary colonisation space. The main filtering material should be as porous as possible and offer a suitable habitat for all strains of filtering bacteria in the form of numerous internal cavities, chambers and tunnels. It is particularly important to ensure that maximum use is made of the available filter volume. Unnecessary empty spaces due to the use of an excessively coarse filter material, for example, mean a waste of potential filtration capacity.

The individual filter stages and their functions

Filter stage Main functions
Preliminary filtration Removal of coarse dirt particles, e.g. dead plant matter, fix excrement, sludge
Main filtration Biodegradation of contaminants / "detoxification" of the water, e.g. degradation of ammonia, nitrite
Fine filtration Removal of fine and ultra-fine suspended matter / clarifying filtration
Special filtration (As necessary) rapid removal of specific contaminants, e.g. nitrate, phosphate, residues of medical agents


Two types of degradation processes lead to healthy water

A fundamental distinction is made between:

  • Degradation with oxygen ("aerobic"): This process renders contaminants harmless via oxidation. Toxic ammonia is converted into less harmful nitrate, for example.
  • Degradation without oxygen ("anaerobic"): Here, contaminants are removed via reduction. Algae-promoting nitrate is converted into harmless gaseous nitrogen, for example. This only works when the environment inhabited by the bacteria concerned does NOT contain any oxygen, however. These "reductive processes" are above all indispensable for good plant growth, as they convert many nutrients into plant-available form.

A good bio-filter material should be designed to enable both processes.