What is the correct use of activated carbon?

(Part 2)

4 Elimination of yellowing and medicinal substances

The most common use of activated carbon is for the elimination of undesired substances from the aquarium’s water. The problem, unfortunately, is that the carbon does not differentiate between desired and undesired substances. In this way, some important elements like iodine are systematically eliminated from the water. Therefore, this is the only valid rule: just use activated carbon in reef aquariums in case of necessity, and especially in the right doses! It is usually placed in the filtering system in bags made specifically for this material. The period of contact does not have an important role, as the water flows quickly around it. The recommended quantities are from about 0,5 up to 1 litre of activated carbon for 500 litres of aquarium water. After one, three days maximum, the activated carbon has to be removed from the water. Long life carbon does not exist! The absorbing surface of the material is quickly saturated by the substances contained in marine water, and then it cannot absorb any more. Once the free surface is filled, the process stops and the exchange begins. The previously absorbed substances are put back in the water and they are replaced by more easily absorbed ones (iodine, for example). After using activated carbon it is essential to dose trace elements again, in particular large quantities of iodine should be re-introduced. The use of this filtering material is however always advisable in case medicinal substances were introduced, in circumstances where it is possible that the fish have been poisoned (heavy breathing), or if the water quality has substantially worsened (yellowing substances). These three situations are the most typical to justify the introduction of such a material into the filtration system. A careless use of activated carbon in reef aquariums is therefore heavily advised against.


5 Biological filtration of the water

Another field of use for activated carbon is biological filtration of water (bacterial colonization). Because of its good absorbing ability, carbon offers to the bacteria that colonize it large quantities of organic substances. The organisms that develop in it (there can also be other unicellular organisms) are benefited by this abundance of nutrients (Reimann 1969). However, not all of the carbon’s surface is made available. The most internal part, in fact, cannot be occupied because of the size of the bacteria themselves, about 1-10 nm. The incredibly small macro-pores (25nm), mesopores (1-25 nm) and micro-pores (0,4-1 nm), that can absorb organic substances, are not reachable by the bacteria. Many aquarists are convinced that active carbon can offer a huge colonisable surface to bacteria. In practice, it has been observed that this belief is not true, and that many other filtering materials are able to offer substantially better performance. However, thanks to its absorbing ability, carbon has a strong attractive force on bacteria. The absorbed substances are processed by the bacteria; in this way the surface is partially cleaned and made suitable again for a new process of absorption. Despite everything, the use of a biological carbon filter is not recommended in a reef aquarium, because of its tendency to absorb trace elements, its predisposition to be obstructed quickly and the resulting lowering of Redox potential. Such an aquarium normally offers enough colonisable surfaces (living rock, flooring). In aquariums of only fish, instead, or in other special tanks (for example mangrove aquariums), the use of activated carbon can still present a valid alternative. Based on the organic matter content, the recommended dose for about 100 litres of water is 1-2 litres of activated carbon. Every two to six months it is opportune to replace a third of the material with some new one. If used in very slow circulation, a carbon filter is also capable to eliminate nitrates. Anaerobic bacteria take advantage of the lack of oxygen, colonizing the more hidden parts of the carbon. This effect, however, is attributable only to the bacteria; carbon itself is not able to absorb nitrates! This assertion too can often be found in books.