$3 or Less DIY Aquarium CO2

You may have seen pictures of thick, lush, planted aquariums, and you’ve asked yourself, “How do they do it?!” Well, you know about light, and you may be aware of fertilizer. But these underwater gardeners have a secret weapon: CO2 injection. “But isn’t that expensive?” Not if you do it yourself! Here’s how…

Jump to Section


Prior to this project, I had never used CO2 injection in my aquariums before. I have read of and seen many examples of how dosing tanks with CO2 (carbon dioxide) can dramatically accelerate plant growth, but it’s not something I’ve really considered before. What I was interested in playing with is using CO2 to help reduce my tank’s pH.

Out of the tap, my water is fairly hard with high pH. That and my preference for sponge filters powered by air pumps, results in my water ranging around pH 8.4+. Just as with very low pH, high pH can cause stress to fish. It can also inhibit the growth of many plant species.

There are a few ways to reduce a tank’s pH. Adding peat moss and driftwood to an aquarium are common strategies, as well as using RO water to aim towards a neutral 7.0 value. But you can also reduce pH by pumping carbon dioxide into the water, creating carbonic acid (H2CO3). Some of this acid will be turned into carbonate compounds by reacting with minerals in the water. This is largely based on your KH reading of your water parameters. The rest of the CO2 will either outgas at the water surface or be consumed by plants and algae.

What exactly do plants do with CO2 anyway? If you’ve taken a high school biology course, you’ve probably learned about photosynthesis. It’s the process by which plants and algae convert light energy into chemical energy in the form of sugar, and it’s the foundation of most food chains on Earth. I won’t go into the details of chlorophyll and electron stripping, but take my word for it, it’s thrilling!

Chemical Equation of Photosynthesis in Plants
Chemical Equation of Photosynthesis in Plants, for your enrichment

As mentioned above, plants use photosynthesis to produce sugar, which fuels their growth and reproduction. The rate of photosynthesis that plants can perform is mostly dependent on two things: availability of light and availability of CO2. There are other factors like nutrition and temperature, but these are the big two. Of these two, the one that is often the limiting factor in natural aquatic environments and in our aquariums is the availability of CO2, because plants use a lot of carbon dioxide to produce sugar and the bulk of their tissues.

On a bright day, a heavily planted pond can soak up all of the CO2 in the water before noon. Once that happens, sugar production drops waaay down. And with that, so does growth rate. Without carbon and sugar, the plant can’t grow new leaves, or anything else really. The same thing happens in our aquariums. Carbon dioxide builds up over night, because plants cannot do photosynthesis without light. During this time, they continue cellular respiration, producing carbon dioxide. When morning comes and the light turns on, your plants will resume photosynthesising, making sugar and growing new stems and leaves. Once the CO2 is depleted, they slow down and can even stop altogether.

Clever aquarists get around this a number of ways. One simple method is to use a rest period in the middle of the day. What this means is turning off the light for a few hours to allow CO2 levels to rise again. Then the light comes on and plants can continue their work. Some other aquarists, on the other hand, use tools like compressed CO2 gas to inject bubbles into the water.

The CO2 bubbles dissolve into the water, providing submerged plants with all the carbon they need to grow day and night (with the lights on). This allows them power their way into thick, lush planted aquariums.

The problem with these CO2 injection systems is that they can sometimes be pretty pricey. The gas itself is relatively inexpensive, but its the equipment that can strain the wallet. This small system by Fluval retails at about $80. So you can see, it’s not an inexpensive strategy. That’s why we are going to do it ourselves!

The system we are going to create uses two plastic bottles that you can dig out of any recycle bin. I’m using a 2 litre bottle and a 20oz bottle, but the size of the bottles really don’t matter too much. The first bottle is going to be our reactor (Bottle #1). Here is where our yeast will consume sugar and produce our valuable carbon dioxide gas. The second bottle (Bottle #2) is mostly to catch any icky water or goop that somehow might end up in the airline. We don’t really want this stuff to end up in our tank, so better it end up in Bottle #2. Also, this bottle will serve as a kind of bubble counter.


Active dry yeast and two bottles
There’s very little needed to make a DIY CO2 system.
In the end, I opted not to use the flow control knob.
One cup sugar and one packet of active dry yeast. Banana for scale.

Ingredients and Expenses

In this tutorial, I assume that you have access to a few basic tools. These would be: scissors or knife, hot glue gun, and water dechlorinator. Now for the ingredients and materials. If you’re starting completely from scratch, this is what you need and your estimated costs:

3 x Red Star Active Dry Yeast Packets – $1.50 for three packets

2 x Plastic Bottles (any size) – free from the recycle bin!

3ft Plastic Airline Tubing – $2.99 for 25ft

1 Cup Sugar – $2.00 for 4lbs bag

Total: $6.49

OK, so you’re actually spending more than $3 up front, but that’s only if you are starting completely from scratch. And with three packets of yeast, you can have three separate batches from one shopping trip. That’s… $6.49 ÷ 3 = $2.16 each! Plus you have plenty of sugar and airline hose for your other hobbies… weirdo. If you don’t want to spend money on sugar, maybe the next few times you order a coffee, grab an extra sugar packet or two. I’m not saying clear the place out, but if you normally don’t take sugar, maybe now you do. If you’ve been in the aquarium hobby for a while, you probably already have some spare airline hose laying around. So your expense may look more like this:

3 x Red Star Active Dry Yeast Packets – $1.50

2 x Plastic Bottles (any size) – free from the recycle bin!

3ft Plastic Airline Tubing – $2.99 for 25ft

1 Cup Sugar – $2.00 for 4lbs bag

Total: $1.50 ÷ 3 = $0.50 each!

Fifty cents per batch! Now that’s cheap. It’s even cheaper if you or someone you know is a brewer or baker and has some yeast in the cupboard.

DIY CO2 Reactor Step-by-Step

Before beginning this project, make sure to read all of the steps and read my warnings below. Okay, let’s get started…

Step 1 – Cleaning Bottles

Before you begin, you’ll want to make sure your bottles are cleaned and free of any weird residue. You don’t really want anything other than yeast to grow in the bottle. Otherwise the process will be less efficient and possibly stinky/unhealthy. So make sure you rinse out your bottles thoroughly, but avoid using soap, because we don’t want there to be any soapy residue leftover either. If you really want to ensure sterile conditions, use hot water and either white vinegar or hydrogen peroxide to sterilize the interior of the bottles. I didn’t do this, but have at it Hoss. Don’t burn yourself, and remember to rinse well afterwards.

You’ll also want to remove any labels so you can visually monitor the progress of your yeast culture.

Step 2 – Jump Start the Yeast

Right now your yeast is having a time out. We want to get it going again so it can get right to processing that sugar and producing CO2. In a clean cup or bowl, empty a single package of the active dry yeast. Next you want to take some warm (not hot), dechlorinated or bottled water and pour it over the yeast. You don’t need to use a lot of water, just enough to completely cover it. Mix the water and yeast around until it is evenly distributed. Put this aside to begin “waking up.” You want to use water that has no chlorine or chloramine in it, because this can inhibit or even kill your yeast. As an aquarium hobbyist, you probably already have some dechlorinator handy. If not, you can use plain bottled water instead of tap water.

Yeast water
Here you can see a mixture of water and yeast.

Step 3 – Connect the Tubing

Before you can connect the tubing to the bottles, you need to punch holes into the caps. Please do not stab, rip, burn, or otherwise injure yourself! Wear protective gloves and eyewear, and make sure a fire extinguisher is nearby. If you need an adult, get an adult!

bottle caps with hole markers
Mark the spots where you plan to make your holes.

What I did was first mark on cap #1 and #2 where I planned to make my holes. Cap #1 only needs one hole. This is the cap that will go on the reactor (Bottle #1). Cap #2 is going to have two holes, one to connect to the line coming from Bottle #1 and one that will go into the aquarium. Then I used the tip of my hot glue gun to melt the three holes marked on the caps. You can use a hammer and nail or a drill, whatever you have, but again don’t hurt yourself!

hot glue bun hole punch
I used a hot glue gun to melt/punch three holes into my caps.

Now you are ready to connect the tubing. You need a short length of tube, in my case it was about 10 inches or so, to connect into the one hole on Cap #1 and one of the holes on Cap #2. For Cap #1, push the tube in from the outside of the cap until it is about an inch into the hole. For Cap #2, push the other end of the same tube in from the outside until it reaches to the bottom of the bottle.


The end result should allow you to stand the bottles next to each other without the tube kinking.

Next, take a long length of tubing and push one end into the remaining hole of Cap #2. Push it in only about as far as you did for the hole in Cap #1. We don’t want either of these super short ends to be in the water. This is going to be the tube that runs from the reactor system to your aquarium, so make sure the rest of it is long enough to stretch from where you’ll be keeping the reactor to your aquarium.

Lastly, you’ll want to seal the areas where your tube enters the bottle caps. I used hot glue around the joins to make an airtight and watertight seal.

Step 4 – Mixing your Ingredients

Okay, so you’re yeast has been marinating, your bottles and tubing are all set, now you can begin the chemistry! Measure out one cup of sugar. I used ordinary granulated sugar, because it is cheap and was on hand. You can use whatever sugar you have access to. Next, add another two cups of dechlorinated or bottled water to your sugar mixture. Mix it up until the sugar is as dissolved as you can manage. Next, carefully pour the sugar water into Bottle #1. Pour your watery yeast into Bottle #1 also, and fill the the rest of the bottle up to 3/4 full with dechlorinated water. The water you use in this step can be warm, but it should not be hot. Hot water may kill your yeast. Plug up the bottle with your thumb or finger and give it a good, hardy shake. You’re trying to dissolve the remaining sugar and distribute the yeast. Lastly, fill up Bottle #2 to 3/4 full with dechlorinated water.

Step 5 – Puttin’ It All Together

Now you’re ready to put your caps on the bottles. Screw Cap #1 onto Bottle #1 and Cap #2 onto Bottle #2. Make sure the shorter portion of the tube in Bottle #2 reaches down into the water, and make sure the smallest ends of the tubes in Bottles #1 and #2 don’t dip into the liquid of either bottle. Now you’re ready to rock and roll! After a while you’ll begin to see bubbles starting to from on the tip of the tube under the water in Bottle #2. For me, this took almost two hours. My yeast was a bit old, and it probably was mostly nonfunctional. You may have quicker results, but if not, don’t give up. It’ll work itself out. Once you see bubbles, you’re now ready to connect your reactor to the aquarium.

final reactor set up
Here you can see the two bottles, side by side. Soon they’ll be pumping out the gas!

Connect the longest stretch of tubing from Bottle #2 to a check valve and an airstone in your aquarium. The check valve is not totally necessary, but as with any air tubing going into the water, you should have one to prevent an accidental siphon from occurring. The airstone is also not necessary, but you’ll get more effective bubbles with one or with a diffuser.

Tubing all set up with airstone
Nevermind the green spot algae epidemic… I’m working on that!

So What’s Going On?

The yeast is consuming the sugar for food. This is called cellular respiration, and its byproduct is carbon dioxide gas (CO2). The gas fills Bottle #1 and travels through the first tube into Bottle #2. There is a little pressure that builds because of the weight of the water in the second bottle. This allows a bubble to build up and be released every few seconds. The gas then builds up in Bottle #2 and in the airline connecting it to the aquarium. Just like in the airline connecting the two bottles, there is some resistance from the weight of the aquarium water on the line. As the yeast digests more of the sugar, more pressure will begin to build up, until there is enough to overcome the weight of the water pushing inwards. That’s when you will begin to see bubbles coming out of your airstone/diffuser. Once you see bubbles coming from that end, you’re injecting CO2 baby!

How Long Does It Last?

It’s hard to say exactly how long this batch will last. In other tutorials I’ve seen online and on YouTube, a rough estimate is about a month per batch for this size. Changing the amounts of yeast and/or sugar will determine how long it will last. What happens is either the yeast will consume all of the sugar and starve, or it will produce enough byproduct to create a pH drop, which eventually kills the yeast. So after a few weeks you may begin to notice the bubbles aren’t coming as quickly as they used to. This should be your sign to begin gathering your materials for the second batch. You can start over with a new set of bottles or clean out your old ones and reuse them.

Alternative Methods and Formulas

I found out pretty quickly that my initial measurements produced way more CO2 than I needed for my tiny 3 gallon aquarium. Within about twelve hours of setting everything up, the system was putting out more than 1 bubble per second! That’s a lot of CO2 for such a small volume of water. So, in order to prevent my plants, snails, and scuds from perishing in an acidic wasteland, I pulled the airstone out and allowed the bottle to run its course out of the water. Based on what I’ve read online, I believe my first set up would be more appropriate for something like a 40 gallon planted tank.

I posted about my experience on my Instagram feed, and a few of my followers offered some awesome advice. Here are some of the suggestions that they made:

After reading some peoples’ advice, I decided to try again with a much smaller reactor. For my second attempt, I used 2 teaspoons of sugar and 1 teaspoon of yeast. That amount seems to last approximately 24 to 36 hours. At peak production, the yeast was putting out maybe 1 bubble every three seconds. With this method, it may be wise to “top off” the reactor bottle once a day or every other day. I also like the idea mentioned of using a blended rice mixture as a source of sugar. The reasoning is that refined sugar is too easily processed by the yeast. Using rice, means it will take longer to breakdown and produce CO2. I think I’ll try that in the future.

Here is what my current, smaller system looks like (minus the tubing of course):

Safety Notice!

With this project, we are dealing with gas under pressure. Gas under pressure is no joke! Your yeast will produce carbon dioxide gas continually whether there is an outlet for it or not. If there is nowhere for the gas to go, pressure will build, and build within the bottle or system. This means you can never (NEVER) completely close up the system. Don’t put a tight-fitting lid on your yeast bottle and do not puncture or damage the walls of the bottle. There is the potential for your yeast bottle to catastrophically fail if the gas has nowhere to go. You do not want pressure to build in your bottles. If you feel that too much gas is being produced for your tank, take the airstone out of the water and move the whole thing to a well ventilated area. Do not try to completely close up the bottle, thinking you are just going to put it on hold for a little while. That’s not how it works. You’ll end up creating a steadily increasing pressure chamber that could potentially fail. DO NOT DO THAT!

Yeast fermentation produces more than just carbon dioxide gas. You may notice a sulphurous smell coming from your system. This is likely hydrogen sulphide, a toxic gas. The quantity of HS thats likely to be produced by a small system should be harmful. Despite this, you should not intentionally inhale any of the fumes coming from your system. Always maintain adequate ventilation im your workspace when using this method or working with compressed CO2 systems.

You should also keep in mind that mishandled CO2 injection can result in mass fish death. For this reason alone, you may want to opt for the pricier option of buying a proper kit. Know that when you follow this or any DIY project guidelines, you are taking on risk of failure.

On a less dangerous note, be careful if/when you try to open the bottles. Have you ever shaken a soda bottle and had your friend open it as a joke? Imagine that, but with stinky yeast water. Yeah, it’s gross. If you need to open it for any reason, do so slooooowly like a soda bottle that you just dropped on the floor. Probably best not to open it at all if you can.

Research Sources




YouTube.com (playlist)


What do you think of this article? Was it helpful? Let me know what you think in the comment section below.

Leave a Reply

Your email address will not be published.