I have a couple of 5, 10, and (1) 16 gal tank I am going to order some very expensive guppies (from Adrian HD). I use the Tetra (I think) filters from walmart on all except the 16 gal, which is Fluval U2. All of the Tetra's come with the cartage already with the carbon in them, and I beleve the Fluval has two cartages of carbon in it. I have read and heard that you should not use carbon at all. That it delays the true cycle and is just something you have to keep replacing. Can someone point me in the right direction here. I have done a search on "use carbon" and "do not use carbon" and "use of carbon" and nothing has come up. Can someone please just point me to a link or something, as I am sure this is a basic question.