Long time lurker, first time poster here! Since there seems to be a lot of chatter about DIY co2 reactors and such I thought I'd share a write up on my own contraption that has worked out great so far. This diffusion method relies on a modified needle wheel impeller and can be built with a minimal investment in materials. In fact, if you've been doing this whole aquarium thing for very long at all you've probably got 99% of this stuff in your "parts bin."
Additionally, if you've got a SunSun HW series "outside filter," it makes building the diffuser even easier, and you'll finally have something useful to do with the "surface skimmer" attachment!
1. Some method of Co2 supply. Either a pressurized tank, or a yeast generator.
2. Maxi-jet powerhead. I'm sure other brands may work, but I'm basing this write-up on my use of the Marineland Maxi-jet and all fitment/compatibility with all the other components. The ones they sell now include several strainers. You will need one of these.
3. Flexible aquarium plumbing as included with most canister filters. Just a 6-8" length of "extra" tubing will do.
4. 3/16" ID airline or Co2 tubing.
5. Top Fin "Mist" air stone. These are the small ones you can buy at just about any "big box" pet store. They are sold in 3 and 6-packs and are about 1/2" in diameter. Unlike most other air stones they will fit inside the flexible tubing without restricting flow.
Constructing the Diffuser
1. First you will want to modify the impeller on the Maxi-jet into a homemade "needle wheel." This will reduce flow somewhat, but will do a much better job of mixing up the co2. Simply line up the tabs on the bottom of the input/output of the powerhead and it slides right off. The impeller's right there and is retained by a magnet so it'll just pull right out. The blades are a semi-flexible rubber. Using an exacto knife, remove small sections of each of the blade along their lengths. Be careful not to cut all the way through or you'll be starting over with a new one! The end result should look like figure 1.
2. Reassemble the powerhead and attach the 6-8" length of canister filter tubing to the input of the powerhead. It should fit snugly. See figure 2.
3. Affix the 3/16" ID output line of your Co2 generator/tank to the "mist" airstone and insert the stone and line into the flexible tubing mentioned in step 2. How you do this is up to you. You could cut an access hole in the strainer that came with the powerhead and simply run the line through there or try any method you like. If you have a SunSun HW series filter, you know that it came with a pretty useless surface skimmer that you probably sealed off. The fitting for the skimmer is a perfect means to attach your Co2 line as it will fit the flexible tubing on one end and the strainer attachment on the other! You can see it in the third picture.
4. Attach the strainer to the bottom of the tube. This keeps anything from getting sucked up into your powerhead. You could also use a pre-filter if you wish. See the fourth picture.
5. Submerge everything and start your powerhead/needle wheel diffuser.
Depending on your bubble count, a 7-up effect is highly possible.
Hope you guys enjoyed this! I borrowed most of the ideas from things I read on this forum. Your comments/suggestions/improvements would be most welcome. Thanks!