You need a diffuser. If one small bubble at a time is rising to the tank, then the co2 is not saturating the water at all. One of the best diffusers for DIY co2 is placing the line underneath a powerhead or wave maker. Reason being is that the pressure isn't good enough to push the co2 through a regular diffuser. When the bubbles go through the powerhead propellers, it chops them up into micro bubbles.I have the 2 bottles going right now. One is on a 20g High and one is on a 20g Long both are very heavy planted and the plants have been growing well for a few months just wanted to give them a jump start.
On diffusing I just allow the bubbles to float to the top no diffuser in the water. With the bubbles being tiny (about 1mm round) they go up pretty slow.
I'm new to the Co2 systems and I saw some places said I didn't need a diffuser so trying it without one for now. I have other tanks with the same type of plants not running Co2 so I can maybe compare the tanks after a week or so. I know it isn't a perfect match but should be close enough to see a difference.
you should just go with a paintball setup now that you see the difference. you can get most if not every thing off of ebay or amazon. I went with a 24oz tank off of ebay same as the diffuser and regulator aquatek mini!! all super worth it and its cheap in the long run and more reliableI did try the chop stick as a diffuser and I'm pretty happy with the micro bubbles. And I spent some time looking at my tanks and I can see a difference in the plants after 2 weeks. Very glad I gave the DIY Co2 a try.
Couple of questions on the paintball setup.
How much does a refill cost?
How often do you have to get it refilled?
How many tanks are you running on one bottle?