No carbon will work as efficiently in a bag as it will run as a block in a reactor. As far as efficiency thou, I believe you want to use a lignite carbon in a bag due to its larger pore structure for free flowing water to pass thru. The Rox 0.8 has a VERY FINE pore structure. Lignite also will work best in a bag as it will create a lot of dust if it were to start tumbling in a reactor, where in a bag, it is pretty much just sitting in a sump or whatever and not doing a whole lot of moving around.