My advanced guide to manage the CO2 addition inside the aquarium

Written by: Marian Sterian
MasterLine Founder

With this article i will cover the benefits of using CO2, the variables that influence the accumulation, how to measure it, what's the optimal concentration, how to keep it stable, how to dissolve it and many others

Benefits of using CO2

Carbon dioxide (CO2) is an essential element for aquatic plant growth, and adding it to an aquarium can have a positive impact on the overall health and aesthetics. CO2 addition can improve plant growth, oxygen production, and even help balance the pH of the water.

Aquatic plants require CO2 to conduct photosynthesis, the process by which they produce their own food using light, carbon dioxide, and nutrients. Without adequate levels of CO2, plants cannot grow properly, which can lead to stunted growth or even death.

One of the most significant benefits of CO2 addition to an aquarium is improved plant growth. With sufficient levels of CO2, plants can grow faster and healthier, which can create a more vibrant and lush aquarium. Additionally, a dense plant population can also help reduce algae growth by absorbing excess nutrients that would otherwise fuel the growth of algae.

Lush growth under non limiting conditions

Another advantage of adding CO2 to an aquarium is increased oxygen production. During photosynthesis, plants release oxygen as a byproduct, which can help maintain healthy oxygen levels in the water.
In strong light, high CO2 concentration and non limiting nutrient values, you often see the plants pearling. This is a very good sign and when it happens it's because the oxygen that the water can hold is at maximum, or saturation. Therefore the oxygen can no longer dissolve and instead it rushes to the water surface in the form of bubbles.
Strong pearling is a good sign because it usually means that the aquarium is doing well, algae growth is at minimum or even non existing, bacteria activity is strong, the water is clean and clear. You should always aim for strong pearling and when it doesn't happen, try to think what is wrong with your aquarium and try to fixt it.

Hemianthus Cuba pearling

Finally, CO2 addition can help balance the pH of the water in the aquarium. The pH of the water can have a significant impact on the health of aquatic organisms, and CO2 addition can help maintain a stable pH level. When CO2 dissolves in water, it forms carbonic acid, which can help buffer changes in the pH caused by other factors such as fish waste or plant respiration.
Most plants prefer a slightly acidic water with a pH between 6.0 and 6.5. At this pH range, nutrients such as nitrogen, phosphorus, and potassium are generally more available to plants.

Variables that influence the accumulation of the CO2 inside the aquarium

Aquarists are having a hard time managing the CO2 addition because although they don't make any changes to the CO2 flow, the concentration inside the aquarium fluctuates. This can lead to a series of problems like poor or stunted growth, deterioration of the older leaves, no pearling and algae growth.
To better understand what contributes to the fluctuation of the CO2 levels, we need to take into consideration the following aspects:

Water movement
- high currents determine the CO2 to evaporate faster. For example after adding a pump or after cleaning the filter, the currents are higher and less CO2 will accumulate
- strong water movement at the surface also prevents the accumulation of the CO2 inside the aquarium
- cooling fans and air pumps also prevent the accumulation of the CO2
- sumps require a higher CO2 flow because the water passing thru all the elements evaporates larger quantities of CO2
- surface skimmers have a similar effect to sumps
- water evaporation can determine the movement at the surface to become higher, leading to lower CO2 concentrations in the aquarium

Plant mass
- plants can also make a slight difference. When your aquarium has more plants, the CO2 consumption will be higher

CO2 bottle pressure
- if you are using a one stage CO2 regulator, when the CO2 in your bottle is running out, the pressure will also drop. This determines the valve inside the regulator to contract and allow more CO2 to pass thru. The result is an increased working pressure which in return will increase your CO2 flow as well. During this period, the CO2 inside the aquarium will also be higher.

How to measure the CO2 concentration

There are a few different methods you can use to measure the CO2 concentration inside an aquarium. I will explain the ones that are most popular:

Drop checker

A drop checker is a small device that sits inside the aquarium and constantly measures the pH level of the water by using a pH indicator solution that changes color based on CO2 concentration. CO2 dissolved in the water, evaporates in the drop checker and this is how the solution changes its color. The reading has a delay of about 2-3 hours, meaning that if you read the color at 12 o clock, it will show you the CO2 concentration that you had in your aquarium 2-3 hours ago.

When using this method, you need to take into consideration that CO2 bubbles from the diffuser can get trapped under the drop checker and determine the solution inside to change color. This doesn't mean that you have the same concentration in the entire aquarium. To prevent it from happening you need to put the drop checker in a position, away from the CO2 bubbles.

For a proper reading i am recommending a drop checker that has a white background. Make sure you change the solution inside monthly or else it might indicate a wrong color, especially if water from the aquarium gets inside it.

pH meter

A pH meter can be used to measure the pH level of the aquarium water. You will also need to know the KH level and use the below pH/KH chart to determine the CO2 concentration.
Although the pH/KH chart method is not so accurate, it can help you make an idea about the approximate CO2 value.
You need to take into consideration that the pH meter needs to be calibrated constantly in order to give you a good reading.

pH/KH chart to determine the approximate CO2 concentration

The above chart is slightly different than others. I believe that in modern aquaristics, the proper CO2 concentration should be between 30 and 50 ppm.

pH controller

This is a device that is similar to the pH meter. It uses a pH electrode which sits in the aquarium and reads the pH value. Based on the KH value, you set the desired pH level in the pH controller in such way that the CO2 concentration will be between 30 and 50 ppm. The pH controller will control the CO2 addition with the help of a solenoid valve keeping the pH value as stable as possible. For example if your KH value is 4, in order to have 40 ppm CO2, you set the pH in the pH controller to 6.5.

The pH controller also uses the pH/KH chart method and the resulting CO2 concentration is an approximate value.

When using the pH controller you need to make sure that your aquarium has a stable KH. I am not recommending using the pH controller in aquariums with aquasoil and/or rocks. Aquasoils release acids determining the KH to drop and a part of the rocks used as hardscape release carbonates which increase the KH.

The optimal CO2 concentration

In most of my aquariums i try to make use of at least one method for measuring the CO2. Using the drop checker is the main one. The color that i am aiming for is a lime green. I believe that this is a non limiting value. My plants grow with thicker stems, bigger leaves and are more strong and healthy under such a CO2 concentration. The pearling is also very strong.

In my opinion, this is the best CO2 concentration (40-50 ppm)

Beside the drop checker, in some aquariums i also use pH controllers. Here i make sure that the KH is stable and that the pH electrode is calibrated monthly. In other aquariums i also use a pH meter and test the KH if i have doubts that the drop checker is not reading well.

Having too much CO2 will determine the plants to grow faster and stretch, with longer internodal lengths, and less red pigments, especially in lower light. On the other hand, having a lower CO2 concentration will determine the plants to grow slower, more compact and with more red pigments. I am not suggesting to go below 30 ppm CO2 as that might be a dangerous zone for more CO2 demanding plants. Usually, when the CO2 is not enough, you get stunted tips.

How to keep the CO2 as stable as possible

Maintaining the CO2 concentration stable is an important factor. Once the plants get used to a higher CO2 value, they will not like it when it will suddenly drop.

When talking about stability, you need to take into consideration all the above mentioned variables.
In my case:
- i try not to have water movement at the surface (the oxygen is produced by the plants)
- i clean my current pumps every 2 or 3 weeks to keep the flow stable
- when the filters get dirty the water flow slows down and after cleaning them the flow is high again. This is causing a current fluctuation inside the aquarium. I observe the drop checker color and adjust the CO2 flow if needed
- i am not using electric skimmers which can clog fast and create a fluctuation. Instead i am using skimmers connected to the inflow and i make sure the water inside them is at the same level. Believe it or not when the water level inside the skimmer is 4 cm below the aquarium water level, more CO2 evaporates compared to the situation when the water level inside the skimmer is 1 cm below the aquarium water level. I also remove any leaves stuck in the upper of lower part of the skimmer
- during the hot days i don't allow the water level inside my aquariums to drop too much. I top off with osmose water when needed
- because i am not using dual stage regulators to keep the working pressure stable when the CO2 is running out, i observe the regulators weekly and if i see an increase in the working pressure which has an impact on the CO2 flow, i lower it to the previous value. If you choose to do this, take note that after replacing the CO2 bottle, you will need to increase the working pressure back to what it was
- i always observe the drop checker and make the necessary changes when needed
- pH controllers are of great help for CO2 stability but you need to make sure that your KH is stable. You also need to calibrate the pH electrode monthly to make sure it gives a proper reading. If you are not using a refillable pH electrode, you need to replace it every 12-16 months as after this period it will start to give bad readings despite the calibration

How to dissolve and how to manage the addition

There are all kinds of ways of dissolving the CO2, some of them easier to be used but less effective and some more complicated but with a greater dissolving rate. I will write a few words about the most used ones.

CO2 diffuser

This is the cheapest and most popular way of dissolving the CO2 in the aquarium. It doesn't require any difficult installation. For me this is not a good option because any equipment that needs to be introduced inside the aquarium spoils the aesthetics of the layout. Beside this, adding CO2 thru a diffuser is not economic. The dissolve rate is low and a lot of CO2 is wasted. Practically all the bubbles that reach the surface are lost and you will need to refill your CO2 bottle more often. Another downside of using a diffuser are the bubbles that get blown by the currents in the entire aquarium. They might also get trapped under the drop checker and give you a false reading of the CO2 concentration.

CO2 atomizer

Atomizers are somehow similar to diffusers but they are a bit more expensive. They are mounted inline on the return hose of your canister, preferably as close as possible to the canister. I am recommending this mounting to improve the dissolve rate of the bubbles traveling thru the hose. Good atomizers give smaller bubbles, just like a mist, which have a better dissolve rate than diffusers. They also have the advantage of keeping the aquarium more clean without too much equipment. They require a higher working pressure than diffusers: around 1.5-2 bars. The CO2 mist inside the aquarium doesn't have such an unaesthetic appearance.

CO2 reactor

This is my personal favorite way of dissolving the CO2. If you are using the right model and you have proper conditions, it has the big advantage of dissolving 90-95% of the CO2. Of course not all external reactors will do this. The model that i am using is the one in the below picture.

I am recommending to install it on the outflow of a filter that has at least 1200-1500 rated lph. Good flow dissolves the CO2 faster and better. The 10" variant is the best because if the flow inside it is high, the bubbles will not be pushed to the bottom of the reactor and towards the aquarium. It's important for the bubbles to stay inside the reactor as long as possible in order to be dissolved by the water passing thru it.

It doesn't need any service if you are not adding any sponge or bio balls inside it. In fact, the reason why i am recommending the 10" version is to avoid the servicing as long as possible. Sponge and other filter media inside, will clog as time is passing by, and they will need cleaning.

The main advantages of this way of dissolving the CO2 are:
- a very high dissolving rate
- the impact on the filter flow is minimum
- it doesn't spoil the aesthetics of the aquarium
- Ii doesn't need high pressure and servicing
- no CO2 bubbles inside the aquarium
- it's cheap to be made

Now that i have told you most of my experience about the CO2 inside the aquarium, i will also write a few words about managing the addition.

There are a few ways that you can do it:
- non stop: you just adjust the needle valve on your CO2 regulator in such way that the drop checker has a similar color to the one i have posted above. This way, the CO2 will run 24/7 and the concentration inside the aquarium will be stable thru the night and day. It has the disadvantage that you will need to refill your CO2 bottle more often
- thru a solenoid valve: for this way of addition, i am recommending that you put the solenoid on a timer and set it to turn ON 2 hours before the light is turning ON. It can turn OFF at the same time as the light or one before the light turns OFF. You will need to adjust the CO2 flow in such way that when the light turns ON, the color of the drop checker is similar to the one above. You will refill the CO2 bottle less frequent by using this method of addition
- thru a pH controller: you test the KH of your water and set a pH inside the controller to have a resulting CO2 between 30 and 50 ppm. You do this by using the above pH/KH chart. It has the advantage of keeping the CO2 stable and refilling the CO2 bottles less frequently. If your KH is fluctuating from using aquasoil or rocks inside the aquarium, this is not a good way of adding the CO2. pH controllers are good only when the KH is stable. You also need to calibrate the pH electrode monthly and replace it every 12-18 months if it's not a refillable electrode. Use a drop checker as backup

And i believe i have covered most of the aspects about using CO2 inside the aquarium. I hope that you will find this information useful and that it will help you achieve better results.