Carbon is a matter of preference. Some run it some dont. It can be beneficial though. It does remove things that other means wont. I recomend running it as i believe in not being able to have too much filtration. You place the carbon in a bag or sometning to hold it and run water through it. It can be placed anywhere you can run water through it. It doesnt "mix" with the water, but instead absorbs things in the water. It needs to be replaced periodically. You should use a good quality carbon made for aquariums.