How to Make a DIY CO2 Reactor for Your Planted Aquarium

Aquarists interested in planted aquascapes will usually consider carbon dioxide (CO2) at one point or another. CO2 is as essential to plants as oxygen is to animals. While fish and invertebrates provide low levels of the gas it takes supplementation to create vibrant planted aquascapes.

CO2 systems can be pricey but there are a number of DIY CO2 systems that can be put together for a few dollars. And chances are, you already have several of the required materials!

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Materials Needed for DIY CO2 Reactor

Putting together the reactor is a simple process and all of the required materials can be found at a general merchandise store like Target.

• Scissors
• Knife
• 1 or 2 Cleaned Soda Bottles
• Aeration stone
• Check Valve
• Aquarium-safe Silicone
• Airline Tubing (15 feet should be more than enough)

Aquarium safe silicone is essential because other brands may leach noxious chemicals if exposed to water and we don’t want these getting into our tank ecosystem.

If you’re going to create a dual bottle CO2 Reactor then you’ll need the following as well:

• 2 instead of 1 Soda Bottle
• T-Way Connector Valve (T-Splitter)

In the image above, I’ve included all of the materials for a dual bottle system suitable for aquariums 30 gallons or larger. If your local merchandise store doesn’t have the aquarium specific components they can be purchased easily through Amazon!

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Setting Up Your DIY CO2 System

Step One: Using the Knife or Scissors punch a hole into the cap of each soda bottle.

Don’t be too forceful as the plastic will give suddenly and you may end up with a hole too large. Simply puncture a small hole and then twist the knife, slowly widening and creating a circular hole for the airline tubing.

Step Two: Cut the airline tubing at an angle, making it easier to insert into the cap. Then, thread the tubing into the soda bottle cap as pictured.

The airline tubing can extend about an inch into the bottle but does not need to penetrate too far. The length of the tubing depends on whether you’re creating a single or dual bottle setup.

A single bottle setup should extend from where you plan to stow the bottle to the aquarium itself. This first line for a dual bottle setup only needs to be 6-8 inches long; just long enough to reach above the second bottle!

Step Three: Take your aquarium silicone and cover the edges of the hole on both sides of the cap to create a new airtight seal.

Then, give the silicone 5 to 10 minutes of rest time to set properly. If you have a smaller aquarium and aren’t creating a dual stage CO2 reactor you can skip to Step Six as the line will run from the bottle to the aquarium!

Step Four: Follow Steps 1 through 3 to create a second reactor bottle for your larger setup. Once finished, you can join the two bottles using the T-Splitter as shown below.

The T-Splitter channels CO2 from both bottles into the aquarium via the next airline tube.

Step Five: Cut the third piece of airline tubing and attach it to the T-Splitter as shown below. This piece will stretch from the Splitter all the way into your aquarium, so measure properly.

Step Six: Find your Aquarium Check Valve and orient it so that the arrow is pointing in the direction of CO2 flow (from the bottle towards the tank). Cut the line running from the T-Splitter or single soda bottle to the tank. Attach the Check Valve anywhere in between the bottles and the tank, ideally closer to the bottles so it’s easy to hide or replace.

The purpose of the Check Value is to prevent water from leaving your aquarium and entering the line if CO2 pressure runs out. Likewise if you need to remove the cap from either bottle, the Check Valve ensures flow only works in one direction.

Step Seven: Where the tubing ends, attach your Aeration Stone. The Aeration Stone will help break up the large bubbles of CO2 into a fizz that dissolves faster.

One major issue with DIY CO2 setups is keeping the bubbles submerged long enough for the CO2 to dissolve. When placing your Aeration Stone within the aquarium, find places where strong currents can help the bubbles stay submerged longer, such as the flow of a powerhead or filter.

You can also place the Stone near the intake of a canister filter, which will thoroughly mix CO2 into the water before sending it back into the aquarium.

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The Final Product

Once it’s all finished, your DIY CO2 setup should look something like this:

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Yeast/Sugar Recipe

For the DIY CO2 reaction itself, we can use a simple recipe:

• 1 Package of Dry Baker’s Yeast per bottle
• 2 Cups of Sugar per bottle
• 1 Teaspoon of Baking Soda per bottle

Our first step is to activate the yeast by placing it and a tablespoon of sugar in a ½ cup of warm tap water (slightly above room temperature is fine).

Mix thoroughly until the water is slightly bubbly and wait 10 minutes. The yeast will awaken and begin feeding on the available sugar.

While waiting for the yeast to activate, add enough warm water to fill the soda bottles ¾ of the way. Add the rest of the sugar and baking soda into each bottle and shake vigorously to mix.

Once the yeast is activated you can add it to its new, sugar-rich home, and tightly reseal each soda bottle.

Over the course of hours the yeast cells will continue to feed and divide. As a byproduct they create enough CO2 that the excess gas will bleed into the line and into your aquarium, feeding your aquatic plants.

How long they generate CO2 is somewhat inconsistent but can range from 1½ to 3 weeks before the system needs fresh sugar and yeast. An advantage of the dual DIY CO2 system is that you can time things so you’re only changing one bottle every 7-15 days.

The amount of CO2 generated isn’t as high as higher end planted aquarium designs. However for the budget-minded and DIY project-lover, yeast-based CO2 systems are one of the easiest ways to enrich your aquatic plants!