I was an avid user of carbon for many years. I abandoned it about four years ago after becoming convinced that its desirable actions were outweighed by the undesirable. A carbon substrate is best at binding other organic molecules, which are also comprised of carbon. So, the good news is that carbon will adsorb organic contaminants which are discharged by the fish during excretion. The bad news is that these organic contaminants are consumed by the bacteria comprising the biofilter as a necessary carbon sorce in their routine metabolism. Many/most individuals who have abandoned carbon report that their aquarium water has taken on an "earthy" smell. Nothing unpleasant...just a bit different in character. I would have to agree with this. There are also several reports indicating a correlation between the use of carbon and an increased incidence of "hole-in-the-head disease". Presumably, this is attributable to the removal of something from the water which augments the integrity of the slime coat. Finally, I am not aware of any report from an aquariast which indicates a correlation between the discontinuation of carbon and a decrease in water quality. Therefore, at best, carbon would appear to be a waste of money and occupies valuable space within the filter, which could be used for additional mechanical or biomedia. Two weeks ago, I used activated carbon in a reactor to remove medication from my aquarium water to make it easier to reestablish my biofilter. Other than that, my personal view is that it has essentially no utility.
