How to Build a DIY Yeast CO2 Reactor

Step-by-step guide for building a yeast CO2 reactor.

For a low-tech planted tank of 30 gallons or less, a DIY yeast CO2 reactor can be a simple, cost-effective way to supplement CO2 without the investment or headache of a high-tech setup. It’s therefore very popular for smaller planted tanks, especially for aquarists who are just getting into the planted tank hobby.

These systems are inexpensive to build and relatively easy to maintain, although the yeast culture mixture needs to be refreshed periodically. For tanks 10 gallons and larger the risk of overdose is minimal, but for nano tanks, we’d advise using a smaller bottle and trying the setup out before adding fish, as an overdose is technically possible, and it’s difficult to monitor or control the CO2 output from this setup.

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What you will need
Before beginning, gather the following materials:

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  • Drill
  • Approximately 10 feet of silicone airline tubing
  • Scissors
  • Sealant such as silicone caulking
  • One-way checkvalve
  • A means of diffusing your CO2 (see below)
  • 2L soda bottle
  • Sugar
  • Active dry yeast
  • Measuring Spoon and cups
  • Funnel
Building your reactor

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Once you have acquired the necessary materials, you’re ready to build your reactor:
  1. Cut your tubing the proper length to run into your tank. Make sure to leave an extra few feet of slack in case you need to adjust the positioning of the reactor later.
  2. Select a drill bit that is slightly smaller than your airline tubing. Use it to drill a hole in the center of the soda bottle lid, and fit the tubing in as snugly as is possible, putting only about 1 inch of tubing through the lid.
  3. Place the sealant on the bottom side of the lid, and allow 1 hour for it to cure. Repeat this process with the top side.
  4. Allow sealant to cure for 24 hours before continuing.
  5. Place one-way check valve in the line, about halfway between the reactor and point of input for your tank.
Diffusing CO2 into your tank
There are several means of diffusing CO2 into your tank from your DIY reactor, some of which are much more efficient than others:

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  • Run the tubing through your filtration intake if possible. This is an easy and efficient means for CO2 diffusion, but may not be feasible for some setups.
  • A commercial CO2 reactor or diffuser, which can be obtained online or at your local fish store.
  • As a last resort, an airstone. This method is not ideal, as it is inefficient and most of the CO2 will not be dissolved into the water.
Making and maintaining the reaction mixture
A DIY yeast CO2 reactor produces CO2 as a metabolic byproduct of yeast fermentation of sugar. There are a few different ratios of sugar to yeast, and you can experiment to find what works best for you, but we recommend the following to maintain an active, steady state of CO2 production:

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  1. Measure out 2 cups of sugar, and place it in the bottle using the funnel.
  2. Add ½ tsp yeast to the sugar
  3. Fill the bottle with cool to lukewarm (not hot!) water, leaving a 3-4” gap between the surface of the water and the top of the bottle.
  4. Shake well to mix.
  5. CO2 production will begin in about 24 hours.
  6. Shake mixture every few days to keep it agitated.
  7. Replace mixture every 3-4 weeks
Adding more yeast will result in higher CO2 production, but the sugar will also be consumed more rapidly, such that the mixture must be replaced more often. This could also lead to a CO2 overdose, especially for smaller tanks. Lukewarm water is recommended to help encourage yeast growth, but make sure that the water is not hot, as this will kill the yeast!

Tips and tricks

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This is a relatively simple system to make and maintain, but there are a few pointers and things to avoid:
  • Never leave an undrilled cap on a bottle containing the reaction mixture. CO2 production is relatively rapid, and the bottle will explode within a few hours, resulting in a huge mess.
  • Do not restrict the flow of CO2 by kinking or crimping the tube, as this could also result in an explosion.
  • If using an airstone or diffuser, periodically remove and clean it.
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