For carbon dosing to work, you need a skimmer. The "carbon" is food for bacteria. The bacteria flourish, and in the process consume nitrates and phosphates. You now need a means to get all this excess bacteria out of your system along with those nasty nitrates and phosphates. And that's where the skimmer comes in. The skimmer is your "export" device
Without the skimmer, the bacteria will die within your tank, returning the phosphates and nitrates back into the water.