I've been using a 402 power head with a gravel tube stuck into the out flow leading to a the actual gravel tube itself like the one in Steven L's link for months and it does an absolutely terrific job of dispersing the CO2. I get a very fine mist, very little of which reaches the surface. I'd have to say that 95-98%is the rate of absorption. And the price is right. Power head $15, gravel vac. $4, Aqua Clear Mini sponge $1, complete.
HAVE DISCS - WILL TRAVEL