Aquarium Filtration & Aeration

Filters are used in aquariums to keep the water clear, and to remove the unwanted debris.  There are basically three different methods of filtration – mechanical, chemical and biological.  Filters may be fitted either inside or outside the aquarium, and be operated by either compressed air or electrically powered water pumps.

Methods of Filtration

Mechanical – this is simply the removal of the waste from the water, where the water passes through some kind of filter and is trapped.

Chemical – the most common form of chemical filtration is activated carbon, and this works by dissolving the waste in the water.  This is great, but it often cannot be used if the aquarium is being treated with medication, as it will remove it from the water.

Biological – this works with no moving parts, no filter, and needs no maintenance except for a periodic stir of the gravel.  It works by passing air through the gravel, and the oxygenated water encourages bacterial growth.  The bacteria in the water then breaks down the toxic substances into less harmful substances.

Types of Filters

Undergravel Filters – this involves having a series of plates at the base of the aquarium under the gravel, and water is drawn through it.  This uses the biological method.  The gravel will need to be cleaned occasionally using a ‘vacuum cleaner’ to make sure the debris is not excessive.

Submersible Filters – these are small filters which sit inside the aquarium, and work mechanically.  These are especially useful for breeding or hospital aquariums.

Power Filters – these provide both mechanical and chemical filtration, and tend to hang off the back of the aquarium.  These are especially popular in the US.

Canister Filters – these are usually placed under the aquarium, and provide mechanical and chemical filtration.  These are suitable for larger aquariums.

Diatron Filters – these are highly specialised filters which are used to ‘polish’ the water, so they are only used on a short-term basis.

Your choice of filter will depend on the size of your aquarium, the fish you will have in the aquarium, as well as personal choice.  I recommend discussing filters in more detail with your local aquarium stockist if you are unsure as to the right choice for your aquarium.


In many cases the choice of filter may provide enough aeration to the aquarium, but if this is not the case, additional aeration is required.  Aeration has the effect of enlarging the aquarium’s fish-holding capacity.  By agitating the water surface it adds more oxygen to the water, while decreasing the amount of carbon dioxide in the water.

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