i use two fans - one fan for cooling the light and another for exhaust. i have my carbon filter inside my grow area connected to the exhaust fan so all the air coming out has to go through the filter.
if you're using only one fan, you'd probably have to attach the carbon filter to the exhaust fan on the outside of the grow area. so with only one fan, you'd attach ducting to an intake hole attached to one side of the reflector. duct work coming out the other side of the reflector to the exhaust fan blowing out into a carbon filter. but this would do absolutely nothing to bring in fresh air and expel stale air.
i would highly recommend two fans (plus some oscillating fans inside the grow area too).