it's a good idea to add the carbon pad after treating the tank with meds but it has a habit of offloading what it has adsorbed after a while. You should remove the pad whenever you add meds of course.
it is reputedly good for giving the water a final polish but I would save your money and stick with the standard foam/fibre filters (as well as the EhfiMech/EhfiSubstrat) unless you need to remove meds