Carbon dioxide (CO2) is vital for plant photosynthesis. Aquatic plants (just like land plants) extract carbon from carbon dioxide to produce sugars and oxygen. The carbon mass in the dry matter of the plant is around 40 – 50%. The main problem, though, is that aquatic plants do not have constant access to unlimited CO2 compared to land plants. As a result, in the best-case scenario, it limits their growth.
In a low-tech (low-light) tank, CO2 is often the ceiling that keeps the growth rates in a tank. Therefore, to solve this problem, aquarists invented different ways and methods to provide additional CO2 in their planted aquariums.
However, when you have a low-tech planted tank and it looks like the plants are starting to require CO2, it can be so intimidating if you do not have experience setting it up. Do not worry, using CO2 is not that difficult.
In this article, you will learn what is CO2, how to set up a CO2 system, when it is time to add CO2. How much CO2 your plants need. How to add CO2 in planted tanks, ways of measurement, and some other useful information.
Aquarium Plants and CO2
Carbon Dioxide is a type of gas. It can dissolve in water and raise the acidity levels in a tank. The increased acidity causes the pH of your tank to change. It is super important to keep a tank at the right CO2 levels because CO2 is critical to a planted tank. All plants need CO2. This is due to how plants need CO2 to photosynthesize.
In general, the more CO2 you provide the more carbon the plants can use to create energy (glucose/sugar) in the photosynthesis period. A lack of CO2 can cause plants to suffer or grow abnormally. Therefore, CO2 levels being too low can hurt your plants in the tank.
However, too much CO2 can be very dangerous to your fish, shrimp, snails, etc. They can suffocate. At the same time, if there is not enough CO2 in a tank, then algae growth will also increase.
Note: There is no direct correlation between CO2 and algae, it is more about the overall tank environment. When plants are happy and have enough food (CO2) to eat, they grow fast and healthy. As a result, aquatic plants outcompete algae in the tank. Therefore, when the plants struggle in the tank, algae thrive.
Aquarium Plants, Light, and CO2
Light and CO2 are interlinked. The more light our plants have, the more CO2 they will be able to absorb in our tanks. So, they will grow and look better. Even though some aquatic plants may not require a lot of light, it is absolutely obvious that their growth rate will still be way better when they have more access to CO2.
However, we cannot add anything uncontrollably. Your tanks must be balanced in terms of light, CO2, and fertilizers. Having a lot of light in low-tech tanks will not increase the growth rate of your plants. On the contrary, it will be covered with algae in no time. This is extremely important!
That is why by balancing light and CO2, we can increase the growth rate by 3 – 5 times!
Related articles:
- Advanced Guide to Planted Tank Lighting
- Light, CO2, and Nutrients: Why Aquarium Plants Struggle to Grow
When Do You Need CO2 in a Planted Tank?
The amount of CO2 that you need for a tank depends on the amount of light that your tank is receiving. CO2 should not be introduced into a tank at night. This is because plants do not photosynthesize without light.
If the plants are not photosynthesizing, then the CO2 will build up. If you are constantly outputting CO2, then this build up could lead to your CO2 levels being too high.
This will likely not kill your plants because plants can survive at much higher CO2 levels than fish. Instead, you put the critters in your aquarium at risk of suffocation when the CO2 levels are too high.
There are safe practices to follow when you output CO2 into an aquarium tank. You should stop and start CO2 output on a schedule if possible.
The CO2 should stop being outputted into the tank about an hour before it gets dark. However, the plants in the tank will need CO2 for photosynthesis. CO2 should be added about an hour and a half before the tank is exposed to light.
How Much CO2 is Needed for a Planted Tank?
The acceptable CO2 range for any tank is between 10 and 30 mg per liter. The recommended amount is 15mg per liter.
This is especially important for shrimp tanks. According to different studies, the dangerous limit for shrimp is considered 30 mg/l. You can read more about it in my article “CO2 in a Shrimp Tank”.
How to Add CO2 in Planted Tanks
Setting Up a CO2 Diffuser
One form of CO2 that you can buy is pressurized CO2. This type of CO2 is used with CO2 diffusers. The CO2 comes in a pressurized container (Cylinder), which contains an immense amount of pressure. You can’t let the pressure out all at once because it would be way too strong.
For this reason, you need to also have a CO2 regulator. The regulator will control how much CO2 is released at one time.
You can also use a solenoid to automatically switch off CO2 in situations where it is not needed. For example, CO2 is normally not needed at night. This is because plants do not photosynthesize at night.
First, you will want to connect the regulator to the CO2 container. You will then want to connect the solenoid to the regulator. You will need CO2 tubing in order to work as connectors for various parts.
I would also recommend using the check valve. It has a little arrow on it that indicates the direction of the flow. If anything happens, the check valve will prevent the water from flowing back from the aquarium into this system.
Tip: Place the check valve at the waterline. If you put it too far away when the power goes out (which
is going to be every night) water will fill the tube until the check valve. Therefore, when you turn on the system in the morning, this water in the tube will block CO2, until the pressure will be high enough to push it back in the tank. Depending on your CO2 pressure, it can take some time.
For the next step, attach the solenoid to a bubble counter using the tubing. A bubble counter will allow you to measure how much CO2 is being released into a tank at once. You should then attach the bubble counter to the diffuser using the CO2 tubing once again.
Note: Instead of filing a bubble counter up with water, I would suggest using a bubble counter solution. This solution is a little bit heavier than water and it will stay in your bubble counter for much longer. Fill it up until ¾.
After everything is connected, you will want to mount the diffuser in your tank. It should be near the bottom of the tank. Be sure to plug in the solenoid and set the timer. The timer helps you be more precise about when CO2 is released into the tank.
CO2 Diffuser vs Reactor
Instead of using diffusers, another way to provide CO2 is to use the exterior reactors. This reactor is a part of the filter outflow and it mixes with the water from the filter flow before entering the tank.
So, which one is better?
I am not going to overwhelm you with all debates regarding this topic. So, to cut the long story short:
| Exterior reactor | Diffuser | |
| Efficiency | High | Low |
| Calibration | Hard | Easy |
| Filter flow rate | Depended | Does not need |
| Usability | Small to Large size tanks | Small to medium size tank |
| Cost | High | Low |
As we can see, the exterior reactor is more efficient (like 40 – 50 %) but it will cost way more money as well. Personally, I do not see any reason for a beginner aquarist to use it.
Another moment I would like to point out is that when you use diffuser you need to have the smallest bubbles as possible. Large bubbles will rush to the surface making it less efficient. Small CO2 bubbles will be able to move around the tank, so the plant will be able to capture and utilize more of them.
Releasing the CO2 Using a CO2 Diffuser
At this point, you should have everything connected and ready to go. You will want to release the CO2 gas AFTER opening the needle pin just a little bit on the regulator.
Then you can go ahead and release the gas safely.
You will slowly use the pin in order to control the pressure. Start to open the valve further until you see bubbles going into the bubble counter. The bubble counter helps you determine how much CO2 is being released at one time. You should be aiming for one or two bubbles per second.
You can use a drop checker to monitor the CO2 levels in your tank.
Important: Keep in mind that the CO2 levels are read hourly. What you are seeing are levels from an hour ago. Make sure not to add fish until your CO2 levels are under control. Too much CO2 can cause fish to be unable to breathe and possibly die.
Adding Liquid Carbon
First of all, is Liquid carbon as effective as pressurized CO2 systems? No, it is not. Compared to injected CO2, Liquid carbon is insignificant. This is not the same as with regular CO2 dosing.
Using liquid carbon is a weak alternative to get around being unable to add CO2 to a tank. For example, you may have a smaller tank or you can’t afford a CO2 diffuser.
Nowadays, I think that Seachem Excel and the API CO2 Booster (links to check the price on Amazon) are one of the most popular choices.
When you add liquid carbon, you are adding a carbon-based solution. This solution works as a fertilizer for plants. As the plants thrive, your algae levels will also go down.
You will need to constantly dose the aquarium with the liquid carbon solution. Depending on the products, the carbon stays in the tank for only 18 – 24 hours. It then disintegrates. So, if you are ready to actually set down the schedule and stick with it, it may be a good choice.
Unlike pressurized CO2 systems where everything is automatic, be very careful with the dozing. It is very easy to overdose and kill all your livestock in planted tanks. This method is not very controllable. It is known to melt plants like Jungle Vals.
Note: People often use such products as Seachem Excel to combat Black Beard algae.
You can read more about it in my article “How to Remove Black Beard Algae in Aquariums”.
Tips for working with Liquid carbon in planted tanks:
- Pour your Liquid carbon from the bottle into a pump dispensing bottle.
- Ideally, you need to use the bottles that can dispense 1ml per pump. It will make your dosing more accurate.
- Use it before the lights are on.
- Dose systematically.
- Do not overdose.
- Before changing the current dose, use the same dose of liquid carbon for a couple of days at least. It will give you a better understanding of how the plants react to it.
Adding CO2 Tabs
According to manufacturers, CO2 tabs are also a viable option for smaller tanks. However, they are not a good choice for larger aquariums. These tabs are easy to use. You should just follow the dosage instructions that come with the CO2 tabs.
Although I have not used them, I have to list them in the article anyway. There is a lot of conflicting information regarding their efficiency.
CO2 tablets dissolve in 1 – 2 minutes in water at normal temperature and stay active up to 48 hours after dissolution. Personally, I do not like the CO2 spikes after dissolution and in my opinion, this is not safe.
DIY CO2 in Planted Tanks
You can also insert CO2 into a tank using “do it yourself” methods (DIY). You can find most of the supplies needed to make a CO2 generator normally.
- First, grab two plastic bottles.
- You will want to drill a hole into the bottle cap of the bottle. You need a drill bit that is just a tad bit smaller than the CO2 tubing size.
- Insert the tubing into the hole. You will want the tubing to go 2 to 3 centimeters (about 1 inch) inside the bottle. You should then add a silicon sealant, in order to seal the tubing and the bottle together. This will make it so that CO2 does not escape unnecessarily.
- Cut the tube length to whatever size you need in order to run it to the tank.
- Next, you will need to add yeast and sugar. These are the key ingredients in the process. Yeast produces CO2 when it feeds on sugar.
- Take the cap off of the bottle. Add 2 cups of sugar and a half teaspoon of yeast to the bottle.
- Fill the bottle up about 3/4ths of the way with water.
- Put the cap back on and twist the cap. It’s often recommended not to twist it very tight. Letting the cap stay loose allows some CO2 to escape which reduces the pressure in the bottle. You can add the tubing to your filter intake or to another (smaller) bottle in increase the saturation.
Fluctuations of the CO2 in the Planted Tanks
Once you have installed and balanced the system, you do not want to fluctuate the CO2 in your planted tanks. Do not play around with the CO2 needle valve too much. Adjust it to a certain level (bubble count) and leave it this way there for a couple of days.
Observe what’s happening!
There is a direct correlation between CO2 and water parameters. Take some notes regarding how CO2 affects pH, GH, and KH. The point is that the CO2 will decrease the pH. If you lower it too much and too fast you can shock whatever animal you have in the tank (fish, shrimp, snails, crabs, crayfish, etc.)
In addition, decreasing the pH will result in filtration problems. The beneficial bacteria, that break down waste in our tanks, prefer stable pH levels between 7 – 8. When pH drops below 6.5, bacterial growth will be severely inhibited. When pH drops below 6.0, their growth completely stops.
In addition, remember, that after balancing, any sudden changes with the CO2 system, will cause CO2 related algae.
Measuring CO2 in Planted Tanks
The bubble counter is a good way to measure how much CO2 goes into the tank. Although the range of 30 – 60 bubbles per minute, is usually considered to be good for an average planted tank. I would not recommend it right from the beginning.
Depending on the tank size, how heavily it is planted, and what livestock you have, I would recommend starting with less CO2. Do not rush the results!
Using a Drop Checker
The idea of using a drop checker is that the different colors let you know how much CO2 is in your tank. The solution will start out blue. it will then change depending on the amount of CO2 in your tank.
If it doesn’t change from the blue color, then your tank does not have enough CO2. You will need to increase your CO2 output. If it turns green when there’s little CO2.
A change to lime green means that your tank is at 30ppm. This is the level of CO2 acceptable by fish. You are always aiming to get your drop checker to turn green. A change to a yellowish-green means that your levels are too high. You will have to slow down your CO2 output.
To set up your drop checker, you will want to fill the bulb looking portion with the drop checker indicator solution. The drop checker should then be submerged in the tank.
Note: Place it in the water, near the surface. Do not place the drop checker below the CO2 diffuser, it should be above.
There is usually a suction cup portion that allows you to stick the drop checker to the tank wall. You should then wait at least 2 hours for an accurate reading. The solution will change colors depending on the CO2 levels in the tank.
Using Plants to Determine CO2 Levels
It is a good idea to choose an undemanding plant or plants for your tank, such as Monte Carlo. The plant should also thrive when normal CO2 levels are present in the tank.
Note: That is why finicky plants such as Dwarf baby tears, Staurogyne Repens, Ammannia Gracilis, etc. are better to avoid for this method.
Since the plant normally does not need much maintenance, you can assume that CO2 levels are off if it suddenly starts displaying strange growths.
The wrong levels of CO2 will not affect your plant immediately. Look at the plant to see if it is thriving. Also, pay attention to any new growths to see if they are growing in healthy.
A lack of CO2 can be shown by the plant developing a thin stem or leaves. Growth in the plant may also be stunted. The leaves may grow in curled or unnaturally. In addition to curling in leaves, the leaves may also appear to be growing in smaller and smaller over time. A lack of plants and an excess of algae can also show you that your tank has CO2 issues.
When Should We Add CO2 in Planted Tanks?
Turn on the CO2 before the lights come on. The CO2 should start about 1 – 2 hours earlier before the main lights come on. It will allow the CO2 level to build up. So, when the plants ‘wake up’ there will be enough food for them. It is a good idea to have the lighting and the CO2 on two separate timers.
An hour before lights go off, turn off the CO2. During this period of time, the CO2 level decreases in the tank, so you will have a natural equilibrium in your tank.
The Siesta Period in Planted Tanks
The siesta period is a popular recommendation in the Walstad Method of setting up planted tanks without CO2 injections.
However, from time to time, people also try to adapt it to high-tech tanks as well.
Its principle is pretty simple and based on the fact that the plants take up almost all the CO2 by noon. As a result, plants cannot perform photosynthesis.
Therefore, when we set the timer to turn off, let’s say at 3 p.m., and turn on at 4 p.m. our plans will have more CO2 to work with.
Do we have to practice the siesta periods in high-tech tanks? I do not think so. The very idea of a high-tech tank is to provide enough CO2 all the time the plants need it. If we have to jump through hoops, while we are doing CO2 injections, it simply means that we have not optimized the system.
Oxygen, CO2 and Surface Agitation
People often think that if you put more CO2 into your planted tank you will lose oxygen or if you add more oxygen you will reduce your CO2 level.
They are wrong.
It just does not work that way. Oxygen levels and CO2 levels are independent. They do not really affect each other. Basically, the more oxygen you have in your tank the more CO2 you can add to your tank.
Surface agitation is another misconception that can lead you to problems with your tank. People often say that there should not be surface agitation, otherwise you will waste a lot of CO2.
They are also wrong to some degree.
Surface agitation is very important with or without CO2 injections because we need to keep that equilibrium of oxygen and CO2 in your tanks. So that we do not suffocate our fish, shrimp, etc.
Basically, a gentle surface ripple will create more surface area compared to still water because there will be more areas for the gas exchange. Simply do not move the surface too much, like stirring the surface (aggressive agitation).
Surface agitation will keep your tank safe. For example, if you have a biofilm on the surface of aquarium water, it will cause a rapid increase of CO2, which may lead to asphyxiation.
In Conclusion
Before you decide to dive into the high-tech planted tanks, you have to do your research regarding lighting, nutrients, and CO2. It is not possible to create a balanced aquarium by rising only one of these elements. I can assure you that you will fail and will not want to continue the hobby.
Although CO2 will definitely help to make things happen faster, it is not mandatory. In most cases, low-tech tanks can be more than enough. Just take your time and be patient.
However, if you really want a very efficient system, you should use a CO2 diffusor (or external reactor). There is no good alternative to it.
