Here's the recipe:
1 empty 2 liter bottle
some airline tubing
3 cups of suger
1 tspn yeast
1 tspn baking soda
Take the cap off of the 2 liter bottle and drill a hole in it just big enough for the airline tubing to get through, push the tubing in about a quarter of an inch and seal it air tight with aquarium sealant. Let that sit for 24 hours(you want to make sure there are no air leaks). Your fermentation canister is now complete
Now, fill the 2 liter bottle with luke warm water until its about 2-3" from the top. Add the sugar, yeast, and baking soda. Put the cap on and put the open end of the tubing into the tank. Now comes the tricky part. You need to diffuse(or break up the bubbles)the bubbles coming out of that line somehow. I use an airstone, but I know some people who had airstones clogging up on them all the time with gunk, but this has never happened to me. You can get creative with how you do this, I know people who put the line right into the output of there filter, and one of my friends has line running into a piece of driftwood it holds the co2 bubbles there for awhile and lets the co2 diffuse into the water. And remember that it will take 1-2 days for co2 output to peak, and it should continue for about 15 days and then you need to mix another batch up. You should start seeing bubbles within 12 hours tho, they might be slow, but they will quicken. Each batch is different.
Total cost: $7.39
To maximize the benefit of co2 you should not have airstones in the tank and should have as little surface agitation as possible, which means you will have to make sure there is enough plants to keep a good oxygen level in the tank. And do not turn off co2 overnight, as this can cause huge ph swings on a daily basis in your tank. Good luck and remember to watch your ph, as co2 will lower it and your kh.