Activated Carbon Aquarium Filters
Activated carbon is a chemical filter media, which is used to absorb pollutants from the aquarium water.
Heating organic substances such as bone or coconut to very high temperatures produces the media; this is the 'activation' process. The resulting substance, activated carbon, is very porous when looked at under a powerful microscope. The pores in the carbon 'trap' molecules as water is passed through it.
Carbon will absorb a wide variety of substances from water and is very useful as a water 'polisher', removing chemicals and pollutants, which could be harmful to fishes and other aquatic creatures.
In some cases, the use of carbon can be detrimental as it will also absorb aquarium treatments, rendering them useless, and will also remove some nutrients vital to plants.
The length of time for which carbon continues to absorb pollutants varies considerably depending on the level of pollutants present in the water. As a rough guide, carbon should be replaced every three months; if it is left for a considerable time, it may actually start to release some of the pollutants it has absorbed.
Because carbon removes many useful plant nutrients, it is not recommended for continual use in planted tanks. Occasional use can be beneficial and most plants will not be affected by a few weeks of carbon use every few months.
Carbon is particularly useful when water quality drops suddenly, pollutants are at dangerous levels, or to remove tannins from the water (discoloured water)