Activated carbon is very porous. It absorbs toxins.
I'm in the HVAC industry. (Heating, Ventilation, Air Conditioning, Controls)
We use activated carbon on exhaust systems to capture vapors, fumes, volatile chemical vapors, etc. My particular company does a tremendous amount of industrial work in manufacturing plants and process facilities.
Example:
We do a lot of work for the Ball Corporation. Remember your mothers old canning jars? Remember the metal lids? BALL. Well, they also make aluminum cans for Coors, Pepsi, Coke, etc. One particular line in the plant sprays a coating on the inside of the aluminum can--the freshness seal on the inside of the can--it is then run through an oven to cure and bake the lining on. It releases a lot of volotile vapors and thats not good for people and most especially not good around sparks and electricity. BOOM!! We run a 30,000 CFM fan through about 3000 cubic feet of activated carbon filters to trap those vapors. We replace the filters every 60 days at a cost of around $15,000 (our wholesale cost).
In an aquarium, activated carbon can removechlorine, chloramine, heavy metals etc.. I replace it monthly.