Check out the following link. It's great.
I am totally new to C02-ing. Thus far all my lazy...aspidistra...could be bothered with is a two liter bottle with a fermenting C02 mixture and a tube siliconed into the two liter's lid. The tube affixed into the lid is a few inches above the fermenting mixture. From there the tube runs directly into my hang-on-back filter. An airstone that makes pretty fine bubbles is attached to the other end of the tube that's in the filter box.
I remove and and check the airstone every couplefew days to make sure it is not clogged. Also, white gunk accumulates around the airstone every few days...I clean this off with hot water.
To read most descriptions of DIY
C02 systems, my system must be a very incomplete system and most of the C02 produced must be escaping at the surface instead of diffusing into the water. But thus far (three weeks running) I definately see that the plants are benefitting. I do need to make a new C02 mixture, like, today.
When I started this, I did indeed just lower the airstone directly into the tank. This, too, benefitted the plants therein. Honestly, I can't afford a store-bought diffuser right now.
Something may go wrong...I am not suggesting my lazy way as a sure, good, foolproof way. I am just presenting this...because it's what I've done so far.
Very possibly, the plants have just a .00001 per cent increase in C02. But they are more healthy, growing better.
What I HAVE NOT noticed is a lower pH. This is discouraging, because naturally lower pH is one benefit I wanted (I have tetras, and tetras reportedly thrive in 6.5 -ish pH).
Not sure if algae growth has slowed.