My power head seemed to be working just fine, but I wondered how much different using a ceramic diffuser would be. Hooked the tube up to a cheap (less than 10 bucks w/ shipping) plastic/ceramic ebay diffuser. A few hours, pressure was building, no bubbles. Crossed my fingers and went to bed, hoping not to wake up to the sound of a soda bottle exploding.
Next morning, diffuser was working great. 2L bottle was much more pressurized, but still softer than an unopened soda bottle after being shaken. Many times more bubbles (over a several second period, hundreds vs. dozens from just the powerhead), and far far smaller. With the powerhead blowing over the top of the diffuser, the CO2 bubbles stay in the water column for many times longer.
My thinking is that smaller bubbles equals greater total surface area, and staying in the water longer equals more time to diffuse, both of which mean that more CO2 becomes dissolved into the water. Anybody care to agree or disagree?