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Inexpensive Supplemental Carbon Dioxide

by Karen Randall

[Editor's note: This article originally appeared in Aquarium Fish Magazine. It has been reprinted at the request and with permission of the author.]

Carbon Dioxide supplementation need not be complicated or expensive, and give plants an amazing boost in a tank with even moderate amounts of light. (approximately 2 watts per gallon)

There are many variations on the yeast reactor theme, so feel free to improvise. To get you started, here's one method that has worked well for lots of people.

Take a two liter pop bottle or a similarly sized plastic juice bottle. Make a hole in the cap just large enough for a piece of airline tubing. This can be done either with an electric drill, or by holding a nail over a hot burner with a pair of pliers until it is hot enough to melt through the plastic cap. Insert a piece of airline tubing into the cap and glue in place with a hot glue gun. The tubing should just come through the cap, but not go far enough into the bottle that it will come in contact with the liquid below.

Using a funnel, place 2 cups of sugar and 1 tsp. of yeast in the bottle. Then fill the bottle approximately to the place where the neck narrows with luke warm (NOT hot!) water, and shake to mix. Screw the cap back on, and insert the other end of the airline tube into the intake of a power filter or canister filter. The bubbles will be thoroughly mixed and dissolve into the water inside the filter.

Until the liquid in the pop bottle has thoroughly cooled, keep the bottle higher than the tank to prevent a back siphon. After it is cool, it is usually safe to put the bottle beside or beneath the tank. Some people like to use a CO2 resistant check valve to totally avoid the possibility of back siphon.

Within 24 hours, your yeast reactor should be producing enough Carbon dioxide to make a noticeable difference in a tank of between 20 and 30 gallons. If your tank is larger, you may have to run more than one yeast reactor in series. If your tank is smaller, you may need to reduce the amount of yeast and/or add a little baking soda to slow the production of carbon dioxide. In all cases, but particularly if you have a very small tank or very soft water, check your pH frequently to make sure that you aren't dropping the pH to dangerously low levels until you feel confident in your system.

Depending on your tap water chemistry and the warmth of your house, you should find that the yeast reactor keeps producing a fairly good amount of carbon dioxide for about 2-4 weeks. At that point, you can revive the mixture by pouring out 1/2 of the liquid, and replacing it with another cup of sugar and filling it up with water again. If you let the reactor go for too long, and the liquid has a strong alcohol smell when you open it up, or if it has completely ceased producing bubbles, you have probably produced high enough levels of alcohol to kill off the yeast. If this happens, simply empty and rinse the bottle, and start again!
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This page was last updated 26 December 1999