I was using Kent carbon. Basically you can use any carbon that isn't too finely grained or if you use a finer grain just turn down the flow. You don't want the carbon rolling around in there breaking off little bits that might go into your tank. Mostly the use of the reactor increases the efficiency of the media by allowing increased contact with water as you are pushing water up through the media and allowing the water reach all areas of the media vs using a filter bag in your sump.
A lot of people have 2, one for carbon and one for phosban. While I'm running the phosban I will probably go back to running the carbon in a filter sock. I'll see how the phosban works first, if it doesn't appear to be doing anything for my tank, I'll go back to using the reactor for carbon only
