There seems to be some confusion here about the definition of "reactor" and "diffuser".
As I understand it, a reactor is where the CO2 gas is generated in a DIY system, through the chemical interaction of the yeast and sugar. Logically, it is an airtight container with a tube coming out of it, for the CO2 to escape.
A diffuser is a device that mixes the CO2 gas with the aquarium water. Examples are airstones, ceramic gizmos, and the Hagen ladder.