What you will need:
An empty 2 liter plastic soda bottle, sugar, yeast (bakers yeast), luke warm water, air line tubing, silicon sealer, airstone, funnel, drill.
Drill a hole into the cap of the soda bottle to fit the tubing (keep it on the small side for a more secure seal).
Insert one end of the tubing into the drilled hole (make sure the tubing is long enough to reach from the floor into the bottom of your aquarium).
Seal it with silicon sealer and let dry.
Fill the bottle with luke warm water to about 3 inches below the top.
Empty the water into a bowl and mix 1 cup of sugar and one teaspoon of yeast, until they are dissolved.
Put the solution back into the bottle and close with the prepared cap and tube.
Connect the other end of the tubing to an airstone and place the bottle on the floor next to your aquarium and the airstone in the bottom of your tank.
CO2 production should start after 1-3 days. The solution is exhausted as soon as there are no more bubbles. Depending on the mixture, one bottle will last anywhere between 2-4 weeks. Be sure to clean the airstone from time to time to prevent pressure build-up in the bottle due to a clogged airstone.
To assure a constant level of CO2 in your tank you can prepare the new solution just days before the old one is depleted.
It is not necessary to disconnect the reactor at night. Shaking the bottle every 2-3 days does not hurt either.
You can experiment with the ingredients: in general, more sugar will make it last longer and more yeast will make it bubble more. Check on your CO2 levels and make adjustments by trial and error, until you have found the mixture best suited.
Disclaimer: We do not guarantee any results. Build at your own risk. While there should not be any immediate danger, children under the age of 18 should have parental guidance while building and operating above reactor. Always wear safety glasses when operating power tools.