Very interesting articles. Will be some good reading materials over the next day or two. I want to state my point of view. I only use GAC for a week at a time. I really dont use it but a few times a year. I`m not saying that it cant last longer. As your article said CS it really depends on the condition of your water. I would go along and agree that a tank with a good skimmer that GAC would last longer compared to one that does not have a skimmer. I choose to be better safe than sorry and use it for a time frame that I know will work. I choose to see how far away I can stay from the cliff`s edge then to see how close I can get to the edge. Nothing wrong with science and articles. I dont think anyone post more scientific article than I do on this site but we also have to consider experience of others. When folks say that their experience is this or that then that`s what it is. Who am I to discount someone else`s experience. We all know that their`s more than one way to do things in this hobby. IMO experience is just as valuable as science. I do appreciate the articles and will check them out but I will also be considering my experience also.