Activated carbon
The most used chemical filter in aquariums. Removes colours, odors, drug residues and organic compounds from water.
Detailed explanation
Activated carbon is a material with a huge internal surface (up to 1500 m2 per gram) created by millions of micropores. Organic molecules dissolved in water are adsorbed (bound) to the surface of the carbon.
In aquariums, granular activated carbon (GAC) is used inserted in the flow path of the filter, in the sump or in a bag in the filter chamber. The ideal grain size is 2-4mm: fine enough to have a surface but not so fine as to clog.
Charcoal removes yellowish colours (tannins released from wood), unpleasant odors, drug residues after a treatment, terpenes released from soft corals and many other organic substances.
In marine aquariums, carbon should be used with caution. It also removes trace elements useful to corals. Many reefers only use it for limited periods (one week a month) rather than continuously.
The quality of the coal makes the difference. Low quality coal can release phosphates into the water making the situation worse. Brands like ROX 0.8 (Aqua Medic), Seachem Matrix Carbon, and Two Little Fishies HydroCarbon are among the best.
Coal runs out in 2-4 weeks depending on organic load. An exhausted carbon does not release the adsorbed substances (this is a myth) but simply stops working. It must be replaced regularly.
Do not use charcoal during drug treatments: it would absorb the medicine making it ineffective. Use it after treatment to remove residues.
HLLE (Head and Lateral Line Erosion) in tangs has been associated in some studies with the prolonged use of dusty activated carbon. Using good quality coarse coal reduces this risk.