I run a sump, and have found a great way to diffuse CO2.
In my 150g, it has an overflow and sump.
Within my sump there is a fluval 204.
What I hae done is used a plastic water bottle, the 250 ml size (not important) with the pull up drinking nipple (important).
I've reefed out the pull up section of the drinky thingy, filled the bottle with coarse filter material (from my old wet/dry) and popped two holes in the bottle.
To this I've attached the output of the fluval to the drinky thingy, it is a perfect fit, and stuck the CO2 output into the bottle of the bottle through the holes I punched.
The flow of the fluval goes down the bottle, out the holes in the bottom and the CO2 goes up.
I can pump in an much O2 as I want, and the best thing is this is quiet, and hidden in the sump.
I tried the microbubbles in the tank through the return of the sump, and it was just too noisy.