Hard to say what the LFS may have meant. Some are better than others.I was at the LFS and the owner recommend looking into a c02 reactor, anyone have any experience with one compared to a full c02 set up?
But what many of us think of as reactors is an end item that does the final job of mixing the gas into the water. Reactors, diffusers and atomizers are all different items to do that. What comes before is vital as well. You can DIY with bottles and mixing sugar and things to ferment and make CO2. Others use a pressure CO2 with a regulator and equipment to control what you get more precisely.
Deep subject with much to learn??