OK, this is easy and only takes a little minute of your time.
Take a 2 liter soda bottle and make sure that it's clean rinse it well and let it dry completely. Drill a hole in the cap that it the size of the tubing that you will be using and then put aquarium epoxy around the hole and slide the tube in. Make sure that your hose is not too far in the bottle, no more than about an 2 inches in. You don't want to have the hose touching the yeast mixture.
(ew, that doesn't sound right does it!?
)
Now from here you have two options, you can either run it through your filter and let it release the Co2 into your filter and then it will be broken down through water movement, or you can use suction cups and a small bubble stone and attatch it to the side of your tank about half way down, you can go as low as the substrate. The good thing about the last option is that you can see very easily if the Co2 "reactor" is still producing, as where with the first option you are not really able to see the air escaping. I preffer the second method myself. You can allso get small bubble diffusers that are specifically for Co2 reactors and you can find them on ----, or a REALLY good LFS.
: Recipie for 2 liter bottle :
1/8 teaspoon Champagne yeast
2 cups sugar
1/2 teaspoon baking soda - (Not necessary, but makes the Co2 production more even and reduces a spike in production.)
tepid water
Once you see the production of Co2 reducing then you will want to remove the mixture and rinse it well and start all over, this will last on average around 3 weeks.
OR!! You could get a Nutrafin reactor they are around 30ish bucks. They come with a few rounds of mix and it's very easy to use, but you have a big plastic thing in your tank, but of course you could just use the methods that I stated above to release the air into your tank and toss the "diffuser" that they give you.
Hope that helped and Good Luck!