carbon works well, but only if you change it often (every month or so).
it will remove alot of things, meds, ferts, and other gunk that may be in your water.
i use it now only sometimes, ie, once a month, and have it in a fine-mesh bag in my ovewrflow. leave it there for a couple weeks, then remove it again. its great in small amounts, and definately can see an improvement in my water clarity.
others will sayt that it is useless and a waste of time. but thats thier experiences with it.
try it out, with and without, and make a conclusion yourself on how you want to use it, if at all.
hope that helps
Chris