For clarification:
Carbons ability to remove chemicals from the water is limited as stated. It is limited by the amount of chemical it can adsorb. therefore in a very dirty (chemically) tank it may only work for a day or two, in a very clean (chemically) tank it may work of ra couple of weeks.
As said it is unnecessary unless you need to rmove specific items from your water i.e. meds chemicals of some kind etc.
It is not useless once it stops adsorbing chemicals simply because it is a pourous material that can and will house bacteria for bio-filtration However all media will do this including filter floss. the more pourous the media the higher the surface area and the more bio-filtration it will create. Carbon is no more useful at this than a multitude of bio-medias out there, Since Carbon is designed to be a chemical filtration media, it is probably not the best choice of bio-medias available. Therefore there is no need to use it for bio-filtration, and that puts us back to the useless argument quite quickly.
On a side note, despite the ever present myth that carbon will release toxins back into the water this simply isn't true. Aside from the minute amounts of things that might be rubbed off of the outer surface during water erosion, the chemical bond won't be released in normal tank parrameters. Also contrary to popular myth, you cannot re-activate carbon in your oven at home. the temperatures needed to re-activate carbon are well above those of most home ovens, and addditionally controlled oxygen levels are required to properly preform this task. In other words in an oxygen controlled kiln it is quite easy for someone who knows what they are doing to re-activate carbon. in our kithens it is all but impossible. And please do not mess around in the kithen with the broiler and an oxygen bottle. doing so could be harmfull or fatal to your health and your spousal approval rating.
Dave
