To carbon or not to carbon is a continuous topic of vigorous discussion in the fishkeeping community.
Carbon is great for removing meds, toxic compounds (other than the parts of the cycle), metals, tannins, and stuff like that from the water. But in a mature tank, all of that stuff is pretty much gone (or very minimal), and keeping the carbon in can in some cases do more harm than good. Especially if you don't change it often enough -- there's a danger it'll just release all the stored toxins right back into the water.
Plus, carbon will remove some of the metals that plants need to grow on (i.e., iron). But there are carbon-like substances, like Seachem's Renew, that have a similar effect as carbon but don't remove the beneficial metals that plants use.
And of course, you have to remove it when you're putting any meds into the tank for them to be effective.
I usually keep it in, at least in one filter, and change it every 3-4 weeks, but the arguments for not keeping it in all the time have merit too.
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If some one wants to take a second and explain that would be awesome! Everything I read is a bit over my head still. Thanks for all the help thus far. I can't wait to get some fish!!!!!!!!!!!!!!
Thanks, I do change mine according to aqua clear directions. They even tell you to not change all the media at the same time, and when you wash it, do in 
