By Jim Kelly firstname.lastname@example.org
The setups on the my aquaria cost about $5 for supplies that will last a year (this includes 2 liters of Coke that you get to drink). Here's the idea (which is due to Thomas Narten off of internet as far as I know). CO2 dissolves into (and escapes out of) water very quickly, so we need a way to produce bubbles of CO2 and to hold them in contact with a fast flowing stream of water so the CO2 has time to dissolve.
CO2 is produced by yeast fermenting sugar into alcohol, so take a 2-liter soda bottle and fill it with lukewarm water to about 2" from the bottom of where the screw cap would be. Pour the measured water into a bucket and add approximately 2 cups sugar and 1/4 teaspoon baking yeast (e.g. Fleishmann's brand from the baking section of Safeway). Stir until both are dissolved, especially the yeast which is harder to dissolve than the sugar. Pour this stuff back in the bottle and fill to the point it normally would be filled with soda.
This mixture usually lasts about a month before you have to mix a new batch (more sugar makes it last longer; more yeast makes it bubble faster but it will run out quickly). Watch for when the bubbles are no longer produced, at which time you'll have a nasty alcoholic swill left in the bottle which I don't recommend drinking. Keep the opened yeast packets in the refrigerator in the meantime or the remaining yeast will die.
Some people have to worry about the CO2 lowering the pH of the aquarium water, but Davis water is so hard that the pH will hardly fluctuate at all. If your water is soft the watch the pH closely during the first few hours of bubbling, and harden the water with calcium carbonate (crushed coral) in the filter if the pH gets near 6. I have gotten readings consistently above the recommended level on my CO2 test kit (Tetra), with no apparent harm to the fish. When using CO2 you must have a cover on the tank and avoid using accessories which mix air into the water (the plants add plenty of oxygen to the water), as the dissolved CO2 levels will fall quickly. If you have tried growing plants for a long time with no luck, you will be amazed at the incredible growth that results.