Activated carbon removes chemicals from the water and only lasts for a short while before all the carbon is reacted, after this it just becomes biological filtration media to house the good bacteria. You should not have a fresh piece (put in the filter less then 2 weeks ago) of activated carbon in the filter when medicating as it may remove part of the chemicals within the medicine resulting in no or incomplete treatment of the problem. It can be used to remove medication after treatment to stop possible stress that the medication may have on the fish.