Activated carbon has been used for decades to purify water in aquariums and for drinking. As mentioned, it gets a bit of a bum rap from hobbyists with planted tanks because it does adsorb impurities that the plants would use as nutrients. This includes any water soluble ferts added for the plants.
Carbon has a use life that's relative to the amount used and the level of impurities in the water. Often the amount used is so small that the carbon is exhausted very quickly. (e.g. the tiny amount inside most cartridges used in cartridge HOB filters).
Unfortunately there's no way to know when the carbon needs to be replaced.
There may be some slight contamination from non acid washed carbon but I don't really know the extent and if that negates the over-all benefit.
I think most experts say that activated carbon use is probably counter productive in heavily planted tanks, but in non-planted tanks, a sufficient amount of carbon can result in much purer water.
As we're all aware, carbon is often used to remove meds.
I'm experimenting right now. I had a mass of floating Anacharis in my tank but it seemed to wane unless I dosed with Flourish Comprehensive like twice a week. It seemed like I was adding impurities to the water so the plants would filter impurities from the water...what? So I decided, at least temporarily, to remove the plants (went into the turtle pool) and added a bag of activated carbon in the filter.
Last edited by AbbeysDad; 04-25-2013 at 09:23 AM..