Another good method for DIY diffusion is a bell diffuser, I use one of these on my 30gl. Basically, it's the bottom of a 16oz pepsi bottle (cut off) turned upside down, attached with suction cups, and the tubing from the 2 liter reactor is fed under the bell. I have a small powerhead increasing the flow under the bell, I have no probs reaching 30ppm Co2 (along with a separate 2 liter reactor on the opposite side with a limewood diffuser). When I say reactor, I mean the yeast reactor, and when I say diffuser, I mean my method of diffusing Co2 into the water column, there seems to be much swapping of terms when it comes to Co2 injection. If this tank is over 10 or 15 gallons, you will want to use two separate reactors, if it's over 30 you'll be hard pressed to get very good results (ie 20-30ppm Co2) without a pressurized canister. | 
