Filtration in a Fish Tank
Filtration is the process of passing water through a media to remove substances or particulate matter, of which an accumulation would be detrimental to the aquarium and its inhabitants.
Most filters employ three stages of filtration; mechanical, to remove visible particles; biological, to use bacteria to convert or remove harmful organic pollutants; chemical, to remove heavy metals and toxic chemical pollutants.
Of these three the most essential is biological, the other two can be carried out by other methods (e.g. waste removal by siphoning, and the use of high quality water as a source)
In some cases it is possible to achieve a 'balance' between the amount of waste produced by fish, and the uptake of these waste products by plants. In such a balance, it is possible to keep fish in proper conditions without filtration, but it is difficult and only recommended for expert fish keepers. Apart from this rare circumstance, it is essential for all fish to be kept with some form of filtration, and keeping them without must be considered cruel.