Beginner’s Guide to CO2 Injection in the Planted Tank

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DIY? Pressurized? Inline Reactor? Needlevalve? Yeast Generator?  What does it all mean?

If you have recently started a freshwater planted tank, you may be interested in CO2 injection.  This article will cover the basics of CO2 injection methodologies and setup.  It won’t delve deep into any one of them, but it will help you become familiar with a lot of the terms and acronyms that are thrown around quite a bit, and begin to get you more comfortable with the idea of CO2 injection in the planted tank.


First off, why do we want to take something that a lot of folks think is poisonous and inject it into our pristine aquatic environments?  What it comes down to is what plants need to grow, and the equation of photosynthesis.

Plants use the process of photosynthesis to capture energy from the sun, and use carbon dioxide (CO2) and water (H2O) to produce glucose (C6H12O6) and release Oxygen (O2).

So in order for this essential process to occur within the cells of plants, there must be CO2 present.

The good news is, your aquarium will remain more or less at equilibrium with the air, so there will always be around 6-7 parts per million (ppm) of CO2 in your aquarium.  This is a sufficient amount for growing plants in low light conditions. However, as you start to add more available energy to the tank in the form of higher lighting, plants can often start to bottom out on the available CO2.

By injecting CO2, you can ensure there is more than enough available CO2 within the water column to keep plant cells producing.  The benefits are healthier growth of plants, along with a reduction in the ability of algae to grow and reproduce, since the plants are growing to their full potential.  In my opinion CO2 is just as good for algae control as it is for general plant growth, assuming you are injecting it properly and sufficiently.


There are basically 2 general methods:

1. DIY (Do It Yourself):  This involves building a yeast generator from spare parts/bottles.  It is very cheap, but also far more labor intensive over the long run.  It can work very effectively, especially on tanks of 40 gallons or less (of course your experience may vary, and it is possible to setup DIY on a much larger tank).

2. Pressurized: This is the generally preferred method for larger tanks, and for those looking for ease of use and automation.  It does come at a cost, however.


DIY CO2 involves mixing water, sugar, and yeast in a sealed vessel, usually referred to as a reactor, and then allowing the output of the sealed generator to bubble into the aquarium. Yeast are living organisms that undergo respiration (essentially the opposite of photosynthesis).  The yeast uses the sugar to produce energy, water, and CO2.  This CO2 is given off as a gas, and it is this byproduct that we want to get into our aquariums.

Here is a basic diagram of a basic DIY setup.  Basically, a DIY system consists of a pressure vessel (2 Liter soda bottles work great, so do 1L Smart Water containers).  The yeast/sugar/water mixture is placed in the generator, and sealed.  A tube runs from the generator to a gas separator.  This allows floccuated yeast to escape into this container so that any that makes it up the tube doesn’t leak into your tank.  If filled with water, it also serves as bubble counter so you can kind of measure how much CO2 your system is producing: CO2 will bubble out the extended portion of the hose as shown in the diagram, and then escape up the left side, towards your tank, and be diffused (more on that later).

The benefit to running 2 generators as shown in the diagram are a few fold:

1. A single vessel is sometimes insufficient for the size of the tank you are trying to inject… a good rule of thumb is 1 generator for each 10-15 gallons of water in your tank.

2. Having 2 vessels allows you to stagger mixture changes and keep the CO2 concentration at a more consistent rate.  When I ran DIY CO2, I would change 1 of the generators each week… so each mixture lasted 2 weeks, but there was always one that was newer than the other.  When you mix yeast, sugar and water, at first it produces a lot of CO2, but as time wears on the mix starts to fizzle out, so to speak.  Multiple mixes means you always have a fresher mix to keep you more stable.  This is important, as I have found through experience that low or fluctuating levels of CO2 injection actually do more harm than good when it comes to algae growth.  If you aren’t willing to commit to doing mixture changes on a regular basis and you don’t want to invest in a pressurized system, you are better off skipping CO2 injection altogether in my opinion.

Just to give you an idea, here is my DIY system that I made… The generators are on the left and right, and the gas separator/bubble counter is in the middle. The line going to the left leads to the tank. As you can see, the type of container is not all that critical, as long as it can be sealed up gas tight, as the only place you want the resulting CO2 gas to go is through your system to the line in the tank.

This is a great option if you aren’t looking to spend a lot of money and have some very basic DIY skills.  I won’t go into construction details, but you can find dozens of threads, YouTube videos, and articles about how exactly to build one.  It can be done for only a few dollars.



A pressurized CO2 injection system consists of a cylinder of CO2 under pressure with a regulator screwed on top that allows the CO2 to flow out of the tank at a controlled rate. A pressurized system is a great option for larger tanks, or if you want to “set it and forget it” often months at a time depending on the size of tank, your diffusion method, and the size of your cylinder.

With a pressurized system, most setups will have an electric solenoid to electrically control the flow of CO2 (think of it as a light switch for your CO2), a needle valve which makes very fine adjustments to the flow coming from the regulator (you need this as we are looking for very very small flow rates of 1-3 bubbles per second (BPS), and a bubble counter, which allows you to see the CO2 bubbles flowing through a liquid so you know how much CO2 you are injecting.  Word of advice, use something like vegetable oil or mineral oil in your bubble counter instead of water; it doesn’t evaporate. The photo above is an example of what a pressurized system looks like.  I ran this one for several years.

While you can expect to spend $100 or more for a complete setup, the benefits are great:  On a 29gal tank, my 5 pound CO2 canister lasted about 3-4 months.  It only cost around $10 to fill at a local home brewing store.  Local welding supply stores can also usually fill them.  Be sure to keep the cylinder upright during transport and while running the system.

The system is also highly controllable.  While with a DIY system there is no off button, with a pressurized system, you can use the solenoid to control whether or not the CO2 is flowing.  This is important because remember plants are using CO2 for photosynthesis, which also requires light energy.  So when the lights are out, plants are not using CO2.  In fact they are producing it/releasing it through respiration.  So, you can hook this solenoid plug up to a timer and have it work in conjunction with your lights.  The other option is to use a pH controller hooked to the solenoid.  CO2 injection causes your tank’s pH to lower due to the fact that it causes the formation of carbonic acid.  By using a pH controller, you can precisely inject CO2 and keep your tank’s pH stable to the nearest tenth of a point.  It is a nice thing to have, but certainly not a requirement, and is quite a bit more expensive than just using a timer. More on measuring CO2 injection and pH later…


There are many methods of diffusion when it comes to injecting CO2.  It can be as simple as taking the output tube and placing it in your hang on back filter compartment, or sticking an airstone on the line and placing the airstone in the  tank.  However, with each method comes an efficiency of diffusion, and some of the more complicated methods of diffusion are far more efficient at dissolving CO2 into the water.

Ceramic/Glass Diffuser:

A ceramic/glass diffuser is a common method of diffusion of CO2 in the planted tank.  It consists of a porous ceramic disc inserted into a glass fixture.  The CO2 line from your system is hooked to it.  As pressure builds, the pores in the ceramic allow micro bubbles of CO2 to squeeze through and rise to the surface.  Micro is important: The smaller the bubbles, the more surface area of water is in contact with the molecules of CO2, and the more CO2 will be dissolved into the water. They are not 100% efficient though, as some of the tiny micro bubbles will make it to the surface, escaping in the air, and thus not dissolving in your tank.

Inline CO2 Reactor:

An inline reactor is a great method to use if you have a canister filter on your tank.  It keeps extra equipment out of your display tank, and is a very efficient way to dissolve CO2.  It consists of a cylinder with hose barbs for your return line on the canister, and small hose barb on the side to hook up your CO2 system.  Some folks use a media of sorts in the reaction chamber, but it isn’t necessary. You basically need to set it up so that the flow from the canister filter  works against the bubble of CO2 that are rising in the chamber.  Eventually nearly all of the CO2 injected into the chamber is diffused into the water, so it is nearly 100% efficient.  You can find plans to build one of these for your system all over the internet.  Just search “DIY inline CO2 reactor”.

Bubble Ladder:

A bubble ladder allows the CO2 to be bubbled out underneath it, and then the bubble climbs the ladder, allowing for increased contact time with the water before making it to the surface.  You can actually observe the bubble getting smaller as it climbs the ladder as CO2 is diffused into the water. This is a pretty inefficient method though, as a big ol’ bubble of CO2 pops at the surface after it slides up the last rung of the ladder.


There are countless other methods of diffusion, and there is no way I could cover them all.  Some of the more common ones are powered reactors (either in tank or out), CO2 bells, atomizers (in tank), and using a powerhead.

If you use a powerhead, it is usually referred to as the “mist method” if you use the PH to chop up the CO2 into a fine mist.  The powerhead will then blow this fine mist of CO2 bubbles throughout your tank, and some of it actually gets trapped under your plant leaves and can get absorbed directly. This is probably the best method for overall plant health and growth in my experience, but I don’t recommend it as your water clarity will suffer: The micro bubbles get trapped in the water column and make the water look somewhat milky.  Good for plants, not so good for setting up that crystal clear show winning tank!


There are 2 basic ways to measure CO2 (both based on the same principle). When you inject CO2 into water, it lowers the pH. So, if we know the amount of buffers in your water (using a degrees Carbonic Hardness measurement (abbreviated KH), and then we know the pH of your tank, we can use a chart to extrapolate how much CO2 is in the tank. Ideally, we want somewhere between 20-40 ppm CO2 in our tank. I always shoot for 30ppm. Below is what the chart looks like:

So, based on the chart, lets say you have a pH of 6.8, and a dKH (degrees carbonic hardness) of 7. That puts your co2right in the sweet spot of 34ppm. Hopefully you already have a pH test kit… Any pet or aquarium store will sell a KH test kit for 5 dollars or so.

Now, for the ingenious, often preferred way to test your CO2: An aquarium drop checker. It works on the same principal I described above. It measures pH, and indicates a certain color based on how much CO2 you have in the tank. You fill it with water of a known carbonic hardness (KH), and a few drops of pH indicating solution (bromothymol blue). The best value is 4 degrees KH water (you can make this using baking soda and distilled water). With the drop checker filled with 4dKH water and some pH indicating solution, it will always let you know at a glance where your CO2 levels are at. If the solution is blue, your pH is high, and it tells you the CO2 is less than 20 or so PPM. If it is dark green, it is around ~25ppm. If it is lighter green, it will be around ~30ppm co2. If it is yellow, than you are above 35 or so ppm. The drop checker pictured below is reading a nice green color, so the CO2 in this tank is just about perfect. Here is what a drop checker looks like:

Now, mixing up the 4dKH solution (baking soda and water) can be tricky, as it requires a pretty sensitive scale and some very small measurements. The drop checker will function less accurately, but still give you an idea, if you just fill it with some tank water (the directions that come with the checkers tell you to do that anyway… but the consensus based on scientific fact show that the most accurate way to use it is to use a specific water/baking soda solution that is measured at 4dKH — if you aren’t using a known constant for the KH, then the colors become less accurate).  If you don’t want to take on making 4dKH solution, you can often find folks selling bottles for cheap on eBay.  As the fluid in the drop checker only has to be changed when the bromothymol blue has faded (usually 6-8 weeks), a small bottle of 4dKH will last a very long time.

The drop checkers you can get on ebay (search for “CO2 drop checker) for the best price. They range in price from a rediculous $40 for an official ADA drop checker, to a chinese knock off that looks almost the same (and definitely functions EXACTLY the same) for $5-6. Some folks are handy enough to actually build their own for next to nothing, with things laying around the house.

The drop checker is great because it will always let you know how you are doing with CO2, but it does lag just a bit. It will be about 15-30 minutes behind what is actually happening in the tank, but that is close enough to give you an accurate picture of what is going on.


Hopefully this will help to give you a basic understanding to the methods and terms surrounding CO2 injection in a planted tank.  It is by no means definitive or exhaustive, but it should give you an idea of what to look for and be able to speak the lingo when it comes to CO2.  I will say this:  In my opinion CO2 injection is the holy grail to planted tanks.  If you are providing enough light and you have your ferts in balance, it is dramatic the difference that CO2 injection can make.  Best of luck, and as always if you have questions or need help, start a thread in the planted tanks forum — there are dozens of folks who can give you a hand in figuring all this out!

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