I recently set up a 29G planted tank with DIY CO2, and went through a lot of these same questions. After unsuccessfully trying a bell, and not wanting to go through the effort of building a DIY reactor, I settled on the powerhead diffusion method from
this thread.
(I'm also rather new to the boards, so I'm not sure that linking stuff worked. Here's the URL:
http://www.aquariumadvice.com/viewtopic.php?t=87190&highlight=diy+hob+injection.)
I'm am really happy with this method. I've found it to be really effective, and setup is super easy.
First, get your powerhead. I'm using an Aquaclear 20, and the packaging says it pushes 127 GPH. On the powerhead intake, use a sharp knife to cut off one of the bars. (I used a Swiss Army Knife, but any sharp knife would probably do. Exacto, utility knife, whatever.) Position your powerhead in your tank. Run the airline tubing from your CO2 bottle directly into the newly-enlarged hole in your powerhead intake. Just stick the tubing in there; it should fit pretty tight. Finally, turn on your powerhead, and point the flow-director down, so the bubbles shoot toward the bottom of the tank.
That's it!
I'm running a pair of DIY CO2 bottles (juice bottles, 1.7ish liters) on a 29G. Both use the same powerhead. The pH/KH charts tell me that I'm getting around 28 PPM C02. My plants are growing great, and I have no algae to speak of. The best part: the whole setup took less than a half hour, and cost around $30.
I'd recommend this method to anybody. There are two minor problems with this method... and they are so minor that I almost don't want to mention them. First, the powerhead will push around any stem plants growing in its flow, and they won't grow straight upward. Second, the powerhead does release bubbles into the water. If those bubbles are going to bug you, you might want to look into another method.
Good luck, and if you have any questions about my setup, let me know.
