activated carbon is a filter media in itself and used for keeping water clean and removing medication from your water - therefore if its in the filter sponge, i hope you have a hopital tank as this will hinder it.
all you will find is that you will filter out medication - removal of activated carbon is often recommended during treatment for ailments, which is why it is rare to have it in the pad - esp as it looses its effectiveness after approximately 8 weeks and starts realsing what its cleaned up back into the water...
good news is it will help good bacteria though as it provides an ernomous amount of space for them.
the sponge that is on those dish/plate cleaners (new unused ones, not too dense a foam and make sure it is not the cotton types - should look a little plastic - well filter like!) - cut off the scouring part and keep the foam part. cut to size and shape the foam part for a nice cheap simple filter pad.