Not only is the carbon useful at keeping the residual ozone out of the tank, but it's also used to keep the other toxic byproducts out. These are refered to as ozone produced oxidants (OPOs, such as bromate, hypobromous acid, etc.) Take a look at these two excellent articles by Randy Holmes-Farley.
http://reefkeeping.com/issues/2006-03/rhf/index.php
http://reefkeeping.com/issues/2006-04/rhf/index.php