I bought a few of the Sweetwater diffusor stones and placed one in the bottom half of my in-tank sump. The sump is 6" wide at one end of the tank, 24" deep, and 18" front to back. (90 gallon tank) Halfway down in the sump, there is a grid of eggcrate that supports a sheet of blue/white filter fabric. The CO2 bubbles come out of the stone and gather under the fabric, then eventually burp out along one edge. The intake for my external pump is at the bottom of the sump, so the CO2 bubbles run against the water flow. Works well enough that I can make my fish go all wonky with a small twist of the needle valve.
I've also purchased one of the Rio RVT powerheads with the venturi after the impeller. I'm going to swap it out with the Sweetwater stone. My thought was that it would swirl the bubbles in the sump a little better and maybe improve efficiency. I'll see if I can set the needle valve lower once I install the powerhead. If not, then the Sweetwater stone is more economically efficient.
I've added a LOT of circulation to this tank, so letting the CO2 run 24/7 is just wasting too much. So I also bought a solenoid switch from aquarium plants dot com and I'm a little disappointed with it. Although their top gun system shows a switch that can be hard piped with copper fittings because the connections are in the sides of the switch, the separate switch that they sell has the ports 90 degrees rotated so you can't connect it with hard fittings because they would conflict with the body of the switch. So you have to use flexible tubing, which they acknowledge by shipping it with tubing fittings. Which in turn adds four new places for a leak, which it did at first.
Just thought I'd share....