A brief digression on injecting carbon dioxide.

Once the authors of The Optimal Aquarium (1986) had popularized CO₂ diffusion as the high-tech secret to growing spectacular aquarium plants, there was an irresistible demand for a simplified do-it-yourself source for CO_2. About 1993 Thomas Narten posted his fermenting-yeast method on the net, and ta da! DIY CO₂ was born!

As soon as talk turns to aquarium plants today you'll start hearing fertilizer recipes and the pros and cons of carbon dioxide diffusion: if you're feeling like a newbie, don't fret about carbon dioxide diffusers yet, or hunt down and buy expensive special lighting that features spectra exactly adjusted to the requirements of water plants. People were growing plants in aquaria when the only available light was an incandescent bulb, and you can too. Fluorescent light, though, is almost universal now; two watts per gallon has been a common rule-of-thumb, though efficient modern compact fluorescents put out a lot more lumens-per-watt.

I'm a lazy, low-tech guy, blessed with soft water, and I haven't started difffusing carbon dioxide into my aquaria. But for you technomaniacs, the best descriptions of the yeast-in-a-soda-bottle CO₂ generator are both at Bruce Hallman's website: Jim Kelly's "Carbon dioxide, a vital nutrient" and Bruce Hallman's own "Cheapo yeast CO₂ for healthy aquarium plants."

A refinement of the yeast-produced CO₂ system involves the addition of two packets of unflavored gelatin, with the usual sugar, dissolved in enough boiling water to fill the soda bottle about three-quarters full. This is refrigerated overnight to set it, and the remaining water and a pinch of yeast are added the following day. The bottle is attached in the normal way. According to the posters at the Aquatic-Plants Digest, the jello will do two things: 1. it will prevent the yeast from having access to all the sugar at once, and thus prevent the exponential growth stage, thereby cutting the peak bubble rate, and 2. as the yeast consumes the sugar, it will use the gelatin as a source of protein nutrient and slowly eat away at it from the top. The number of active yeast organisms will be stabilized, allowing for a longer, more even CO₂ production. The jello is sort of a regulator--— the more you use, the less sugar will be available at a time, and the CO₂ production will be slower but longer. So you could adjust the recipe to fit your needs. And you could even use cherry-flavored Jell-O! (though that is a more expensive source for gelatin and sugar) as Susan Ziegler did in her DIY CO₂ Trial in Tom Griffin's AquaSource magazine, Aug 2001.

A caution, if you have soft, unbuffered water: yeast's metabolism produces CO₂, which tends to depress the pH in your DIY CO₂ bottle if your water is only lightly buffered. A pinch of bicarbonate of soda can stabilize your brew.

By way of contrast, you might read George Booth's full-scale high tech version, with a tank of compressed CO₂, regulator, controls, and bubble counter, and a version of the now-famous table of the relationship between CO₂, KH and pH-- the absolute accuracy of which is sometimes questioned. Testing reagents count phosphates as though they were carbonates, and humic acids, such as tannins, supplement the carbonic acid of dissolved CO2, and further skew the results.

Supplement your understanding of the carbonate buffer and pH with Chuck Gadd's pH calculator and the chart, set into the context of some useful background science and a couple of corrected misunderstandings of the pH-KH-CO₂ relationship: Chuck Gadd writes very clearly.

Dave Gomberg's "Frequently asked CO₂ questions" are another handy resource. He was recently the publisher of Planted Aquaria, so he knows.