As far is i know the active carbon is not neccessary in the reef tanks as it takes all the trace elements out of water. If your tank contains inverts and corals, you want these elements to be available for them all the time.
Some people don't use carbon filteration at all. Some people put the bag with carbon grains to the place with maximum flow only for a few days. After that the bag should be removed from the system and disposed of. You will need to add trace elements and iodine, as it was removed by carbon during the filteration. I use this method and IMO this is the right one.
Dima.