No, that small black tube on the left is the co2 line, ideally it should be as low down the reactor as possible so it has more flow going against it which slowly dissolves the co2. No microbubbles in the tank and better diffusion. The thing is using a ceramic diffuser is not really the best way to introduce co2 on a larger tank as a good percentage of the bubbles make it to the surface and release the co2 into the atmosphere instead of water, with the reactor set up the way I have co2 cant leave the reactor until it is fully dissolved. The only reason I can think of to have the reactor on the outflow is to keep it from getting full of gunk and fish waste, I have seen some fancy diy reactors though where they use it as a mechanical filtration chamber as well by filling it with either sponges or filter floss.