Ah, carbon. Foundation for everything organic, it’s the building block of life. But as with building blocks of any kind it can be used to build great things, like fish and corals, and it can be used to build, horrible, unspeakable things, like dissolved organics, the forefathers of nitrates and phosphates.
But in a classic case of fighting fire with fire, carbon itself is a key weapon in the battle against unruly dissolved organic carbons and the algae fertilizing nutrients they precede. Carbon, specifically activated carbon, has a chemical affinity for organics, acting like a molecular magnet pulling waterborne contaminants into its chemical embrace. Together with an effective protein skimmer activated carbon plays a key role in keeping aquariums crystal clear and algae free.
The following excerpt from Aquarium Technology discusses an experiment which looked at the efficacy of activated carbon over a period of days and weeks:
Activated carbon is very porous and thus has a large surface area (600 to 1,400 m²/g) and also has a large adsorbtion capacity for dyes and odorous substances. For a long time already this property has been utilized in water treatment technology. Activated carbon can remove heavy metals, drugs, ozone and chlorine gas as well as difficult-to-degrade metabolic residual matter from water. These difficult-to-degrade residual substances have the unpleasant property of yellowing the water after a certain period of time. Among other things, old water can be recognized by its yellow tinge.
What’s the significance of the yellow tinge in the operation of the aquarium? Apart from the visible side-effect of the yellowing mentioned above, a secondary, biological effect might be added. The light produced (at great electrical expense) by aquarium lights is weakened by the yellowing. This filtering of the light can create a significant inefficiency in the aquarist’s attempts to give the aquarium organisms the light they require.
The carbon test described below indicates with which speed the activated carbon removes a certain amount of fulvic acid from the water. Its results can help an aquarist better understand how to use carbon and how frequently to change it out. To conduct the experiment a 20 gallon aquarium was filled old aquarium water with a noticeable yellow cast. Using a small Turbelle (150GPH) water was pumped through a cartridge containing commercially available activated carbon. The concentration of fulvic acid was determined by means of a spectrophotometer. (Incidentally, within the framework of this analysis, pelletized carbons of three different grain sizes were examined and the differences between the three were insignificant.)
Within four days half of the fulvic acid was removed. After seven days 83% percent was removed from the water. After one week the decrease in fulvic acid concentration occurs at an increasingly slower rate. A 100% reduction was reached after thirty days. In conclusion, changing of activated carbon is probably most effectively done after a period of 10 to 14 days.