What is Activated Carbon?
By Andrea Watts - 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
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?
Chemical absorption is a basic principle of
aquarium 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
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
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 may 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.