Carbon Dioxide,

a vital nutrient.

By Jim Kelly jkelly@landau.ucdavis.edu

The importance of CO2 injection cannot be overemphasized for growing beautiful planted aquaria.

Sounds complicated? Actually it's easy and cheap! Aquarium companies sell extremely overpriced CO2 setups costing at least $200 for no frills models. These consist of a high pressure tank of CO2 and a pressure regulator, as well as a reaction chamber where the CO2 is dissolved in the water.

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.

Drill a hole in the center of the top of the cap which is just wide enough to tightly fit a piece of aquarium airline tubing into it, and glue the tubing into place with aquarium silicone sealant. Leave the cap off the bottle to dry for a day. Then screw on the cap and put the other end of the air tube into the intake tube of the filter, so that the CO2 will bubble into the filter. The CO2 may start bubbling the next day, or maybe not for up to 3 days. The bubbles get sucked into the pump propeller and some end up in the filter sponge where they slowly dissolve into the water where the plants can use it for photosynthesis.

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.

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