This is not an original idea, but the original write-up where I learned about this was on another forum, and I can't find it now. It is based on running the CO2 bubble stream through a powerhead to get a much finer stream of tiny bubbles of CO2.
Start with a Hagen Elite underwater filter:
This is a very small filter/powerhead combination, available from Amazon, about 4 inches top to bottom. The bottom half is a filter chamber with a sponge in it. Discard the sponge, and cut a small hole in the bottom of that chamber that will allow you to force the hose barb on the outlet of this check valve in it:
The check valve is made and sold by US Plastics as 1/8" Kynar® Standard Check Valve with Viton® Diaphragm, 1/8" Kynar® Standard Check Valve with Viton® Diaphragm | U.S. Plastic Corp.
This check valve requires only about 1/2 psi to open it, so it is easy to test by blowing into each end. And, it is made of some very chemical resistant plastics so CO2 doesn't degrade it.
Attach the CO2 silicon tube to the end of the check valve that sticks out, attach the assembly to the inside wall of the tank, low in the tank, and you are ready to go. With the check valve located at that spot, water cannot creep back into the CO2 tube, so you get CO2 bubbles almost the instant the CO2 comes on.
This works very well for low concentrations of CO2, for low to medium light, and/or small tanks.