thanx pleco, good articles. here is part of one of them that answered my question.
De-adsorption is another phenomenon that is over-stated in the rumor mills about activated carbon. Again, it is an incomplete statement that is commonly used to described the process. It goes, in one fashion or another, as: don't use carbon because once its adsorption sites are full it will release, or de-adsorb, all the stuff it has adsorbed releasing a large amount of pollutants back into the aquarium. The implication in this sentence that activated carbon works something like a capacitor such that once at its maximum adsorption capacity, it instantaneously discharges all the bad things it has adsorbed is wrong. Carbon does de-adsorb, in fact, that ability is exploited for recycling precious metals. However, in a controlled industrial process, the quick release of the target substance is accomplished by switching the pH of the water. The basic process is to capture the target substance at one pH extreme (very acidic or basic) and then reclaim the substance by switching to the other pH extreme. As stated earlier in this article, these pH values are outside the normal range of aquaria. De-adsorption is not a process to be worried about.
when i was at the pet store, i was looking at a coal based carton of ac made by aquarium pharm for $12/14 oz. according to the article, this is the stuff you want to use. and i do not want to keep carbon in my tank for long periods, i just wondered what would happen, if anything, if you did.