Signs Of Too Much CO2 In Aquarium: What Is It & How To Fix It?

CO2 is among the most important chemical substances for your aquatic community. Not only does it benefit plants, but CO2 can also encourage and stimulate the growth of animals. However, if the concentration of CO2 is through the roof, terrible consequences may transpire for your tank.

So, what are the signs of too much CO2 in aquarium? And what can you do to minimize its negative impact? Keep reading to find out more!

3 Signs Of Too Much CO2 In Aquarium

You can rely either on your fish or other testing methods to understand whether the level of CO2 in your aquarium is suitable or not.

Fish behavior

The most telltale sign of your tank suffering from a spike in CO2 is the decrease in fish activeness. Usually, healthy fish would spend a lot of time wandering around the tank, lurking inside the plants and playgrounds, as well as mingling with others.

However, if the water is lacking oxygen and mostly contains CO2, most fish would try to swim near the water’s surface. This is an attempt to grasp as much oxygen as possible since the current surrounding is not capable of meeting the fish’s needs.

fish rising to water's surface

Another sign of fish subject to too much CO2 is their lethargy. Due to the lack of energy supply, fish would not be able to engage in their daily social activities. This is manifest in labored breaths, sluggishness, and delayed reaction time when it comes to being fed and being played with.

The last behavior from fish that indicates an unfavorable concentration level of CO2 is their erratic swimming patterns. If it is mild, then you will see fish swimming around rather helplessly. But if things get too severe, these fish will gasp at the water’s surface and even try to jump out of the tank.

Drop checker

Should you want a scientifically proven way to measure the CO2 level inside your aquarium, using a drop checker is highly recommended. For those of you who do not know what it is yet, a drop checker is a container with a blue liquid used in CO2 measurement.

CO2 drop checker

Once dipped into the water, this checker will signify the current level of CO2. If the contained liquid turns from blue to yellow, then you would know for sure that your tank has too much CO2 inside.

Given its easy working mechanism, drop checkers are available at all fishing shops and can be purchased at a highly affordable price.

pH and kH ratio

Finally, you can speculate the CO2 levels inside your aquarium through two other metrics: the pH and kH ratio. There are a set of recommended values regarding your pH and kH ratio. If these values are deemed good or acceptable. you would not have to worry about your tank.

On the other hand, should the values of these two be too high and provide an alarming ratio, then your tank is likely to have too much CO2.

Still, it is worth mentioning that this method is not always accurate. Since you have to base your final verdict on two other metrics, it might be confusing trying to determine the correct numbers.

How Do You Regulate CO2 In An Aquarium?

Regulating CO2 in an aquarium is simple enough once you understand the most basic techniques regarding its supply.

First off, you will need to perform a large water change. A 50% or even 70% water change would be best, as it helps to get rid of any residual CO2 inside the tank. Furthermore, it removes dying fish and plants, preventing them from releasing even more CO2 inside the water.

clean aquarium

Next up, check the valve of your CO2 injection system and see whether you can tighten it. Sometimes, the valve is left too open, leaving plenty of space for CO2 to come out.

Once you have narrowed down the entrance, surely less CO2 will be found inside the water.

Last but not least, countering the effect of CO2 using oxygen tends to be highly effective. You can either use air stones or air pumps to oxygenate your water flows.

Not only does this fresh supply of O2 help the aquarium regain its balance, but it also provides your fish and plants with a chance to become healthy again.


1. Does CO2 affect pH in aquariums?

Yes. Generally speaking, the more CO2 your water has, the lower its pH level will be. Given the connection between these two metrics, you must consider all the specs of your aquarium carefully lest you want your fish to survive and thrive.

2. Can too much CO2 cause algae?

No. On the contrary, too much CO2 is one of the most effective ways to reduce the growth of algae.

Algae tend to be the direct consequence of too much light exposure. Hence, your CO2 level has nothing to do with the increasing presence of algae.

3. How much CO2 do aquatic plants need?

On average, aquatic plants need roughly 30 PPM of CO2 to grow sustainably. That said, you cannot apply this metric to all types of plants, seeing how they all have their different needs.

Thus, it is important to grow plants with a similar CO2 requirement together. This way, you can save more time and energy taking care of the plants without having to ponder how to best fit their needs.


Once you have pinpointed the signs of too much CO2 in aquarium, it is time to act on it. Remember, having too much of something can be a problem for both your fish and aquatic plants. Thus, make sure to lower the CO2 level and strike a balance so all parties involved can live in harmony.