Saltwater and freshwater aquariums are an excellent tool for teaching ourselves about nature and the planet we live on. Earth is a biosphere with plants, water, sunlight and living organisms.
In water micro-organisms play a major role in breaking down the waste products of fishes and related toxins into harmless chemicals.
Ammonia which is lethal to fishes is broken down by aerobic bacteria into nitrite which becomes nitrate.
Plants utilize nitrate and fish eat plants so the cycle is perpetual.
In a closed water aquarium the volume of water is such that the normal dilution and assimilation of toxins that occurs in nature does not always work. Particularly when the aquarium is small and the inhabitants are many.
Various types of biological filters are used to break down the wastes in an aquarium.
A biological filter can be anything that houses and is conducive too the multiplication of aerobic and anaerobic bacteria. Many filtration devices have been produced to achieve this goal.
Rocks and plants make an excellent biological filter if in sufficient enough quantity to offset the bio-load. They are referred to as “live rock” and “macro-algae” for marine aquariums and are dredged from tropical marine waters for both ornamental purposes and to provide natural filtration. The same applies in freshwater aquariums although freshwater fish are much more tolerant of less than ideal water conditions
The amount of filtration needed is a function of the bio-load of the aquarium. In other words how many fish and other living creatures are housed in the tank.