IMO: Your diffuser needs 30 or more psi to work , you need a needle valve to bring that psi pressure down even more for our co2 use. {Bubble per Second}
A Quote from AquaZilla:
Needle Valve - the most important part of the regulator. The needle valve provides precise control over the amount of CO2 allowed to enter the aquarium; it further reduces exit pressure of the regulator to approximately 1 - 3 psi. It allows you to fine tune and regulate the rate of flow (the number of CO2 bubbles per second) at the desired level. Not all needle valves, however, are created equal. It is important to invest in a quality needle valve for increased precision and to avoid the potentially negative effects of “end of tank dumps”. As the liquid gas in a cylinder nears empty, the pressure in the cylinder will decrease, causing the output pressure to quickly increase and potentially “dump” out of the cylinder into the aquarium. A needle valve will control this “dump” because it functions as the gateway that controls the volume of gas entering the aquarium, it will stabilize the increased output pressure at the set rate of flow (bubbles per second) preventing the CO2 from completely dumping into the aquarium.