If you have a CO2 glass diffuser then odds are you feel like the bubbles are too big and quite often shoot straight to the top of your tank without diffusing into your planted aquarium.
Many glass diffusers do a good job, but they're pretty cheap and as a result the ceramic plate used is not the best. Sure it works fine but the bubbles can be on the larger size than you actually want (relatively). Now if you paid, lets say £100 for your diffuser, the ceramic plate would be of a much higher quality and you probably wouldn't be reading this post. If CO2 bubbles are tiny, not only are they less obvious in your planted aquarium but they also distribute better, after all your sole purpose of having CO2 in your tank is so that it spreads over everything and your plants absorb as much as they can.
With CO2 bubbles that are too big, they have a tendency to shoot to the surface of the tank and not get pushed around your aquarium. To remedy this use an AE Design Needle Valve. Simply insert one end of your CO2 tubing into the Needle Valve, and then insert the remainder into the other side. This would then attach directly onto your diffuser. You would now have total control of your Co2 and you will be able to produce much smaller bubbles than you were before hand. The valve on this piece of equipment is subtle and easy to use.