Here I used a Marinland MaxiJet Powerhead with its output connected to the top of a gravel vacuum tube with the water flow moving downward. I let the output of my CO2 just bubble up from the bottom of the vacuum tube without a diffuser. The CO2 is forced to remain in the vacuum tube constantly rising in the downward flow of water until it completely disolves.
Basically its a modified Bell Method, only with water movement.
And yes, a oneway is used in the CO2 line. But be sure its CO2 rated. Metals will corrode very quick.
Not my design, more info here...