[ming]

Quite technical
Heres some links I stole from the bulkreefsupply carbon FAQ which explains in more laymen terms.
http://www.pets-warehouse.com/carbon.htm
http://www.cee.vt.edu/ewr/environmen.../sketcarb.html
http://www.hallman.org/filter/gac.html
http://www.malawicichlidhomepage.com...ed_carbon.html