Not sure if this was mentioned, but since you're into DIY, you should look into building a co2 diffuser. The amount of co2 that can be absorbed by the water in the time that it takes the bubble to go from the bottom to the top of your tank is pretty small. A diffuser recirculates the bubbles around and around, allowing nearly complete absorption.
Lots of good vids of DIY solutions working on youtube...I like the kind made from glass. They can be anywhere in your tank, even near the surface a little more out of sight, unlike most in the vids which are on the bottom.
YouTube - co2 diffuser diy