Okay, this should probably go under the DIY forum, but since xpistalpetex asked, here is my version of DIY CO2:
Two 1-gallon bottles* (Tropicana OJ) that I alternate. Each has 4-weeks of CO2 production (1/4 tsp yeast, 6 cups water, 2 cups sugar).
(*if you have the output end in a proper diffuser, use a 2-liter bottle to reduce the air space "void")
I have a T valve connecting the two containers and run only one line as output.
The 'make' was simple. I punched a nail through the cap, cut the tip of the tubing to a point, and pulled it through about 1-2 inches with pliers. (The dark color of the contents of this bottle is because I'm trying a cheap/no chemical wine-recipe.)
Check Valve between the T Valve and the output end
The "correct way"* to run the output directly to a powerhead. The CO2 bubbles get sucked into the intake and this provides a fine bubble spray with your waterflow. (*I took the picture this way to show what is recommended, but I actually have mine hooked up to the air suction intake and get a vacuum effect. I can only get away with this because I have so much air space in the 1 gallon jugs.)
Also, I don't use a bubble counter because the amount of CO2 I'm getting isn't near enough to over-dose my tank, but I recommend one for safety (muck guard) for anyone doing DIY and for smaller tanks.