15 Signs Of Too Much CO2 In Aquarium [Causes + 5 Solutions]

Have you ever noticed your aquarium fish behaving strangely or your plants not growing as they should?

If so, you might have too much carbon dioxide (CO2) in your aquarium.

While CO2 is essential for plant growth, excess levels can harm aquatic life.

This article will explore the signs of too much CO2 in an aquarium and how to avoid it.

In short, if your fish are gasping for air at the surface of the water, your plants are melting or turning yellow, or your pH levels are unstable, then your aquarium may have too much CO2. It is important to monitor the CO2 levels in your aquarium and make adjustments as necessary. CO2 test kits are readily available and can help you determine the levels in your tank.

Importance Of CO2 In Aquariums

Importance Of CO2 In Aquariums

Before jumping to the signs and symptoms, let us first understand what CO2 does for your aquarium.

1. Promotes The Growth Of Healthy Algae

CO2 plays a vital role in the production of healthy algae.

The more CO2 you add to the aquarium, the faster your algae will grow and multiply.

Algae is essential for your aquarium because it filters out toxins from your water.

When algae are present, it is also beneficial because it provides oxygen for the fish.

It also helps control the population of other bacteria and microbes in the tank.

2. Helps Regulate pH Levels

The pH level in your aquarium water depends on how much CO2 is present in your water.

The pH level is very important for maintaining the health of your fish.

If your pH levels are not in the right range, your fish may have problems with their skin, eyes, and gills.

This will eventually lead to the death of your fish.

When you add CO2 to your tank, it mixes with the water and forms carbonic acid.

This mixture lowers the pH level of the water which means that it becomes less acidic. As a result, the environment is less harmful to your fish.

3. Promotes The Growth Of Beneficial Bacteria

The bacteria found in the aquarium are very important as they aid in keeping the water clean and prevent the growth of harmful bacteria.

One of the benefits of adding CO2 is that it increases the population of beneficial bacteria in the aquarium.

As a result, the water stays cleaner for longer.

Also, fish need bacteria in order to stay healthy and thrive.

Good bacteria are crucial in keeping the balance of your fish healthy.

Adding CO2 to your tank can help bring back the level of beneficial bacteria in your tank.

4. Promotes The Growth Of Plants

Plants help to purify your aquarium.

Without them, you will not be able to get rid of unwanted chemicals and contaminants in your water.

By supplying plants with CO2, you are giving them an environment in which they can grow without having to worry about contaminants.

Plants like the extra CO2 which provides them with more nutrients. This means that they will be healthier and fuller.

5. Essential For Photosynthesis

CO2 is needed for photosynthesis. Photosynthesis is the process by which plants create energy.

When you add CO2 to your tank, you are helping the plants do their job and create energy.

This is why adding CO2 is a great way to ensure that your plants don’t go hungry.

15 Signs Of Too Much CO2 In Aquarium

Now that you know the importance of adding CO2 to your tank, it’s time to find out what too much CO2 in your aquarium can do to your water, plants, and fishes.

In this section, we will be discussing the 15 signs of excess CO2 in your aquarium.

In Aquarium Water: How Badly It Can Effect

In Aquarium Water

The first thing we notice in the aquarium is the water, and the quality of the water says it all.

By just looking at the water, you can tell if there is something wrong.

For example, if you notice any kind of foam or scum in your aquarium, it could mean that your water is not clear enough.

If you have algae, it could be from having too much ammonia or nitrate in your water.

Below are the possible symptoms of a high level of CO2 in aquarium water.

1. Algae Growth

The growth of algae in your aquarium water means that you have a problem with the quality of your water.

If you notice excessive growth of algae, then you need to figure out the cause and fix it as soon as possible.

A higher level of CO2 may be contributing to the growth of algae.

This means that you might have a green or brown-colored film growing on your aquarium walls or furniture.

Some types of algae can even affect your fish negatively as they can consume oxygen when they are growing.

2. pH/kH Fluctuation

You will see that the pH and kH level in your tank is fluctuating.

This is because the CO2 is constantly being absorbed into your aquarium water.

The carbonic acid from this process lowers the pH and increases the alkalinity.

pH level and the K level are indicators of the quality of your water. If these levels are not stable, it could indicate that the water is not properly balanced.

If you notice that your water has a strong odor, it may be because of these fluctuations. You will also see changes in the color of your water.

3. Murky and Cloudy Water

If you notice cloudy or murky water in your tank, there is likely some type of problem.

Such problems are a lack of oxygen or the introduction of excess CO2.

When your tank is not receiving enough light, algae will grow in the water.

These algae can turn bright red in color and give off a bad odor. It can also cause damage to the fish.

Another common issue with cloudy and murky water is a lack of phosphates.

When phosphates are low, bacteria will build up in the water and make it look cloudy.

4. Water Looks Faded

When there’s an excess amount of CO2 in your tank, you will notice the water looking dull or faded.

This usually means that the aquarium is not receiving enough light, which may mean the need for an extra light source.

5. Slimy Film Development In The Water

The high amount of CO2 can also cause a slimy film to form in the aquarium.

This will appear as a thin layer on the top of the aquarium water, but it should clear up once the CO2 levels are balanced.

This looks really ugly and can ruin the appearance of your aquarium.

6. Bad Smell

This is a common problem for all aquariums, but some are more prone to it than others.

Usually, this is caused by too much nitrate in the water.

If the smell is extremely strong, it can be dangerous for your fish.

You will know the smell because it will be very strong and unpleasant.

In Aquarium Plants: Symptoms To Look After

In Aquarium Plants

Even the aquarium plants will let you know about the excess symptoms of CO2 in the tank.

Let’s discuss some of the common signs: 

7. Yellowing Leaves

The leaves of your aquarium plants may start to look yellow, as a result of too many CO2 emissions.

If it persists longer, then you will notice that your plants will become very sickly.

In fact, it could also kill them. They will most likely have to be removed and replaced with new ones.

8. Bubbles On Leaves

If the bubbles are large and they float on the surface of the water, then you will know that the CO2 concentration is too high for the plants.

It can damage the leaves and even cause death to the plant.

You should lower the CO2 levels in the tank or replace it with a low-CO2 aquarium air system.

9. Stunted Growth Of Plant

You’ll also notice that the leaves of your plants have stopped growing.

If this happens, then you will need to reduce the CO2 levels in the tank.

Once the CO2 level is lowered, you should see that your plants will grow and thrive once again.

10. Browning Of Leaves

The colors of your plants might start to turn brown.

This means that your water has a higher amount of nitrite than it should have.

This also means that the chlorophyll in the leaves is broken down or destroyed.

All these happen just because of the higher level of CO2 in the water.

Related article:

13 Signs Of Too Much Light On Aquarium Plants? And Solutions

In Aquarium Fish: The Behaviour Says It All

In Aquarium Fish

So, we discussed the water and aquarium plants, now let’s watch our fish’s behavior that indicates if the tank has a high amount of CO2 or not.

11. Fish May Gasp For Air

When this happens, it may indicate that the fish are stressed out by the CO2 levels.

Sometimes when fish are stressed out, they may gasp for air.

This can be caused by the fact that the CO2 levels are too high or too low.

It could also mean that you need to adjust the aquarium water parameters.

12. Lethargy and Listlessness

Fish are lethargic when there are CO2 levels in their water that are too high.

They don’t seem to move around and they do not look like themselves.

They are usually sleepy and they appear to be uninterested in their surroundings.

A fish with too much CO2 in the water might feel faint or even pass away.

If your fish become lethargic, this could indicate that they may be having health issues, like anemia. Lethargy can also be caused by stress or illness.

13. Loss Of Appetite

If your fish has lost its appetite and or is losing weight, it may be because of a number of factors.

But one of the most common reasons is that it does not get enough oxygen and a high amount of CO2 is taking place in the tank.

Make sure that the water quality is correct, the light levels are right, and the filter is working properly.

14. Altered Swimming Behaviour

This is a very dangerous behavior to watch out for as it can cause injuries or death.

A symptom of high CO2 content in water is that your fish may start swimming erratically and unpredictably.

Your fish may be exhibiting altered swimming patterns that could be a sign of neurological damage, like paralysis.

Be aware of their location and keep an eye on them at all times.

15. Pale Or Discolored Gills

There would be an obvious loss of color in your fish’s gills when exposed to high levels of CO2.

You should seek the help of a trained fish professional if you notice this kind of symptom.

It may indicate that your fish is suffering from stress or some other disease.

Also if the gills are blue or have a blackish color, the fish is suffering from poor water quality and it needs immediate attention.

7 Main Causes of High CO2 in Aquariums

7 Main Causes of High CO2 in Aquariums

Now that we have discussed all the possible signs of too much oxygen in the aquarium.

In this section, we will be discussing why actually this problem occurs in the tank and other elements that cause the high level of carbon dioxide in the water.

Below are the 7 main causes:

1. Poor Circulation Of Water

It is very important to check out because water circulation is one of the biggest causes of high levels of CO2 in aquariums.

If there is no circulation in your aquarium then you will experience a rise in the level of CO2.

This can be caused by poor design of the filter system and it also can occur due to the fact that there are too many plants or decorations which restrict the flow of water.

It is important to make sure that the circulation of water is always happening in the tank.

2. Overfeeding Fish

If you have overfed fish in your aquarium, then they will start to consume more oxygen than they can process in their body.

Thus, the water will start to become more acidic and the oxygen levels will drop.

This condition may lead to a buildup of CO2 in the water which may also result in high pH and low oxygen levels.

3. Too Much Light

Too much light in an aquarium will result in the death of photosynthetic organisms in the water.

In this case, there will be a buildup of CO2 in the water. 

Thus, it is recommended to keep your aquarium in well-lit conditions to ensure that the CO2 levels remain within normal ranges.

4. Overuse Of CO2 Fertilizers

When you use a large amount of CO2 fertilizers in the aquarium, the pH level will fall and the oxygen levels will rise.

However, if the CO2 fertilizers are used excessively, then the levels of nitrate will also increase in the water.

It is important to know how to use CO2 fertilizers so that they do not harm the environment in any way.

5. Low pH Levels

When the pH levels are too low in the water, it may cause harmful effects on the fish and plants.

This will also result in a lower oxygen level and higher CO2 levels.

It is always recommended to maintain the pH level around 7.8-8.0 for optimal fish and plant growth.

This may lead to a buildup of ammonia in the water which may also cause a sharp increase in the pH level.

6. Overcrowding Fish Tank

The overcrowding of the tank may lead to high maintenance costs.

You should always keep your fish tank clean. The algae should be removed once or twice every week.

If you do not clean it regularly, you will see a buildup of algae in the tank.

The algae will eventually affect the fish’s health.

7. Poor Ventilation

If you do not have proper ventilation, you can expect a buildup of heat in the tank.

As a result, the fish will be unable to regulate its body temperature and will die.

You must ensure that there are proper air vents in the tank.

If you use a heater, make sure you have an air filter installed to avoid any build-up of carbon dioxide.

How To Measure CO2 In an Aquarium

How To Measure CO2 In an Aquarium

Measuring CO2 levels in an aquarium can be tricky, but it is essential to ensure that your fish and plants remain healthy.

Earlier in the article we discussed that CO2 is essential for the growth of aquatic plants and the overall health of the aquarium.

However, too much CO2 can be harmful and even lethal to your fish.

Therefore, monitoring CO2 levels in the aquarium is necessary to maintain the perfect balance for aquatic life.

Here are the steps to follow on how to measure CO2 in an aquarium:

Conduct Water Test

The first step to measuring CO2 in an aquarium is to conduct a water test.

You can use a pH test kit to determine the acidity or alkalinity of the water.

A drop checker kit can also be used to measure CO2 levels.

Understanding the pH Scale

The pH scale ranges from 0 to 14. A pH level of 7 is neutral, below 7 is acidic, and above 7 is alkaline.

A pH level of 6.5 to 7.5 is ideal for most aquariums.

Using Drop Checker Kit

The drop checker kit is a small container that is placed in the aquarium.

It is filled with water and an indicator solution.

The solution changes color depending on the CO2 level in the water.

Blue indicates low levels of CO2, while yellow indicates high levels of CO2.

Analyzing the Results

After conducting the water test and using the drop checker kit, you need to analyze the results.

If the drop checker kit indicates a low level of CO2, you may need to add more CO2 to the aquarium.

On the other hand, if the drop checker kit indicates a high level of CO2, you may need to decrease the CO2 in the aquarium.

Consistent Monitoring

Measuring CO2 levels in an aquarium is not a one-time event.

It is essential to monitor levels regularly to maintain a healthy aquatic environment.

Regular monitoring ensures that CO2 levels are within the safe range and prevents harm to your fish and plants.

Alternative Methods

There are alternative methods to measure CO2 levels in an aquarium, such as using pH controllers, conductivity meters, and redox meters.

These methods are more expensive and complex than using a drop checker kit.

However, they may provide more accurate results.

The Right Level Of CO2 In Fish Tank

The right level of CO2 in a fish tank depends on several factors, such as the type of fish and plants in the tank, the size of the tank, and the water temperature.

The ideal level of CO2 for a fish tank ranges between 20-30 ppm (parts per million).

5 Solutions To Remedy Too Much CO2 In Aquariums

Solutions To Remedy Too Much CO2 In Aquariums

Too much CO2 is a big problem that needs to be addressed immediately.

The amount of CO2 in the air depends on many things, including weather, time of day, and activity in the room.

A properly maintained and well-ventilated aquarium should have no more than 20-30 ppm of CO2.

The following tips will help you get rid of too much CO2 in your fish tank:

1. Reduce The Number Of Fishes

Fish need a certain number of CO2 molecules in their environment to breathe properly. Fish also use CO2 to feed themselves.

If the water contains too much CO2, then the CO2 needed by fish is less, and they are less likely to survive.

Fish require 10 to 15 ppm of CO2, while plants need 50 ppm.

Try reducing the number of fish and plants in your aquarium.

If you cannot reduce the number, try removing some fish and plants and adding other fish or plants instead.

2. Adjust Your Feeding Habits

Many fish that live in aquariums eat a diet of flake food (e.g., flakes).

The CO2 content of this type of food can be quite high.

When feeding flake food, feed it slowly over several days so that it will be broken down and diluted with water.

In general, if you feed your fish twice a day and they are not growing well, reduce the amount of food you give them.

3. Increase Water Circulation

The circulation of the water can play an important role in regulating the pH of the water.

If your water is not circulating, it can be difficult for the pH to stay at its optimum level.

Try adding a filter pump to your aquarium.

This will increase the flow of water throughout your aquarium.

4. Add More Plants

Adding more plants will help absorb CO2 and make the water more alkaline.

However, be careful to not add too many plants or you could end up with a swampy, green aquarium.

You also need to remember that plants take nutrients from the water.

Make sure you provide enough nutrition for your plants.

If your fish are growing poorly, try reducing the number of plants.

5. Increase Ventilation

Ventilation is an important part of keeping a healthy tank.

If your tank is not well-ventilated, then it will get very warm in the summer and cold in the winter.

In the summer, you will have a build-up of ammonia and other waste products in the water.

In the winter, you may have a buildup of salt.

Be sure to open up the hood in your tank once or twice a week to let the heat out and fresh air in.


Can I overdose CO2 in an aquarium?

An aquarium is designed to have an optimal amount of CO2, which should be around 30%. Overdosing with CO2 can cause respiratory problems, so it’s important to keep this in mind when you’re making adjustments.

Can too much CO2 cause algae?

While CO2 is beneficial for fish, too much can cause algae. It is recommended that you only use around 30% of the total amount of CO2 provided by the manufacturer in your tank.

What does CO2 poisoning look like in fish?

Symptoms of CO2 poisoning are very similar to symptoms of oxygen poisoning. In severe cases of CO2 poisoning, you can see fin rot, erratic swimming behavior, sluggishness, and loss of balance. These symptoms are usually a result of a low dissolved oxygen level.

How long should I run CO2 in my aquarium?

The amount of time that you run CO2 will depend on the size of the aquarium. If the aquarium has a large surface area, you can run CO2 for up to 12 hours. If the aquarium has a smaller surface area, you can run it for up to 6-8 hours.

How much CO2 can fish tolerate?

This really depends on the species of fish and how they were raised. Some species of fish have a high tolerance for CO2.


In conclusion, keeping an aquarium can be a rewarding hobby, but it requires attention and care to ensure a healthy environment for your aquatic pets and plants.

If you notice any of the 15 signs discussed in this article, it’s important to take action to reduce the levels of CO2 in your aquarium.

Fortunately, there are various solutions available, such as adjusting the lighting, improving the aquarium’s surface agitation, or adding live plants to absorb excess CO2.

By being aware of the potential causes and solutions of too much CO2, you can maintain a thriving and beautiful aquarium that you and your fish will enjoy for years to come.

So, keep an eye on your aquarium’s CO2 levels, and don’t hesitate to take action if needed.

Recommended article:

9 Signs Of Too Much Oxygen In a Fish Tank [and What To Do]

CO2 in Planted Aquariums: Pros and Cons to Consider