The Use of Activated Carbon in the Aquarium

What is Activated Carbon?

In its original form, charcoal is very light due mainly to its porosity. It contains phosphorous, sulphur and heavy metals – all of which are highly undesirable in an aquarium. It is sourced from the combustion of wood, lignite, bituminous coal, peat, or coconut shells. The activation process involves either thermal (carbonization and gasification) or chemical (with zinc chloride or phosphoric acid) reactions. These processes eliminate all impurities (elements other than carbon), and increases the overall porosity.

Activated carbon is marketed for aquarium use and is available in powder and granules. It can either be purchased in convenient sachets (which are simply placed into the filter), or as loose matter used to fill a specialized compartment. It is very important that the carbon is thoroughly rinsed to remove the dust or residue that is produced during its manufacture.

Why use Activated Carbon?

Adsorption is the basic principle of filtration i.e. the removal of impurities. This occurs when the undesirable molecules are trapped in the pores and outer surface of the carbon. The filter’s performance is linked to the available surface area, and therefore to the porous nature of the material contained in the filter.

There are conflicting theories arising from the use of activated charcoal in an aquarium. Firstly, some people believe that it should be used permanently. The second school of thought promotes the occasional use of carbon. Personally, I believe that generally it is unnecessary to use filtration over activated carbon continuously. It is best used in response to particular requirements, such as the elimination of toxins, medicinal residue, or pigments such as tannin (given off by wood or peat).

Filtration over activated carbon can prove useful in the long term to treat water containing high concentrations of undesirable substances like chlorine, chloramines, alum, phenols and insecticides and pesticides. Reverse osmosis systems are usefully coupled with activated carbon pre-filters. Used in conjunction, they eliminate chlorinated by-products than can damage the membrane. However, they are not very effective in the removal of nitrogenous by-products. Only the rigorous upkeep of biological filters and efficient biological filtration can help eliminate ammonia, nitrite and nitrate.

The Negative Impact of Activated Carbon

Activated carbon can be criticized for its inability to differentiate between “good” and “bad” molecules. It also fails to retain important trace minerals, including those needed by many hard water fish species and many plants. In reality, the adsorption power of activated carbon is dependent upon the different parameters (e.g. pH of the water) or the chemical form under which the element in question is found. The power of adsorption is limited and it losses its fixation capacity after several days, once it has become saturated. Even worse, it can then release the molecules it had previously extracted back into the water. Therefore, it must be replaced frequently; frequency being dependent on the saturation or concentration level of undesirable elements in the aquarium.

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