Active carbon supposedly removes the necessary nutrients from the water that plants need. I don't know this for a fact as I don't use carbon in my filters, but it's something you might want to consider when choosing filter media.
activated carbon is a good medium...but last for a very limited amount of time. It will filter out discoloring agents, and other unwanted substances from your tank's water, but its ability to do that begins to recede quickly, as the tiny pores of the carbon fill up. If you frequently put fresh carbon into your filter, it'll be an effective "cleanser." On the other hand, so long a you aren't overfeeding and putting unnecessary substances into you tank, IMO, there's no pressing need for it and your water will be fine if you just follow regular maintenance, like properly limited feeding, use good-quality foods, do regular water-changes, use good-quality filter medium as mentioned by the others, clean the filter of the tank only when necessary, etc.
I can defiintely recommend Seachem's Matrix as biological media. I've been using it for several years and have found it to work very well. The media never has to be thrown out or discarded after being used for a long time.