Signs Of Too Much CO2 In Aquariums [3 Ways to Tell & How to Fix]

Signs of too much CO2 in aquariums include gasping or stressed fish and a drop in ph levels.

Carbon dioxide is essential for your plants to flourish, but do you know it is toxic to your fish? Yes, but not always, only at a certain level. Therefore, before adding CO2 to your aquarium, you must know the right CO2 concentration.

Luckily, you are at the right place because, in this guide, you will learn the importance of CO2, safe levels, and the signs of too much CO2 in aquariums. In addition, we will show you how to reduce the CO2 level if it goes high.

Carbon dioxide overdosing is one of the biggest causes of livestock fatalities in planted aquariums. Therefore, your job is to find an exact balance where your fish and plants can live happily and harmoniously.

A lush planted aquarium with CO2 drop checker
A lush planted aquarium with CO2 drop checker Owner: James Martin

Importance of CO2 in Aquarium

Carbon dioxide is essential for plants for photosynthesis. Plants produce necessary foods to survive in the presence of carbon dioxide, micro and macronutrients, and light. As CO2 is an integral part of the photosynthesis process, healthy plant growth is dependent on the proper CO2 level.

However, plant species vary in their CO2 demand. Some aquatic plants can grow well without the external addition of CO2 in the aquarium because aquarium water contains some amount of CO2 produced by fish, bacteria, and other livestock.

The amount of carbon dioxide level is around 400 ppm in the air, so floating aquatic plants can readily get sufficient carbon dioxide for them. But in the water, CO2 drops nearly 2 to 4 ppm, which is not enough for most underwater plants. That’s why you need CO2 injection and a reliable way to do it using a CO2 system.

SummaryYou need CO2 for aquatic plants to grow but in a balanced way with the reliable CO2 dosing system

What are the Safe Levels of CO2 in Aquariums?

Safe level of CO2 for fish

You need carbon dioxide for your plans, but a high carbon dioxide level can hamper your fish’s peace, so you have to strike a balance that suits both of them. 

The safe level of carbon dioxide to maintain in your aquarium is under 32 ppm. However, the toxicity threshold of CO2 alters depending on the oxygen level in the aquarium. In a poorly aerated aquarium, fish are more prone to CO2 toxicity.

But if you can maintain proper dissolved oxygen levels in your aquarium, the safe CO2 limit increases.

Related Read11 ways to increase dissolved oxygen in a fish tank 

SummaryAll aquarium fish and invertebrates show stress symptoms when the CO2 level reaches near or goes past 32 ppm. Try to maintain CO2 levels in a safe range for happy and healthy aquatic critters.

What are the Signs of too much CO2 in Aquariums?

#1: Fish behavior

Signs of too much co2 in fish behavior

Your fish can give you the signal of high CO2 levels by their changed behavior. If you are well known about their activities and attitude, you may easily interpret the difference. But if you are a beginner, it’s difficult for you to understand if there is anything wrong or not. 

In addition, all fish will not show symptoms at a time because every fish has its own different CO2 tolerance levels. Also, fish living near the bottom of the tank—such as bottom dweller fish—will show early symptoms rather than fish living in the middle or near the surface.

CO2 injection system
CO2 injection system Owner: James Martin

In general, you should look for:

  • Sluggish behavior/lethargy
  • Lack of appetite
  • Gasping at the surface 
  • Erratic movement
  • loss of consciousness or dead fish

Sluggish or erratic movement is very common to fish when something goes wrong in tank parameters; here the CO2 level. Also, you may observe food palettes floating on the water’s surface even though you gave the exact same amount the other day. 

At increased CO2 levels, fish suffer from a lack of oxygen. They come near the surface and try to gulp air with a wide-open mouth. But don’t get confused with the activities of Labyrinth fish, who used to go to the surface from time to time to take in air; it is their normal behavior. 

If you observe all the fish at a time gasping near the surface, that is definitely a sign of a lack of oxygen, and it could be a sign of a high CO2 level. If your luck sucks, you can see fainted fish or dead fish because of too much CO2 in your aquarium.

Summary: Observing fish behavior can be an excellent way to understand an elevated CO2 level; however, it could be tricky if you are a beginner.

#2: pH & KH relates to CO2 level

Another widely accepted way to understand the CO2 level is using the relationship between pH, KH, and CO2. The pH scale determines how acidic or basic a solution is; here, consider aquarium water. KH denotes the water’s pH buffering ability, meaning how susceptible it is to changing the pH. 

Do you know that carbon dioxide addition can alter your aquarium water pH? When CO2 gas comes into contact with water, it dissolves and produces carbonic acid, lowering the pH. There is a fundamental relationship between CO2 and its effect on KH and pH. 

Aquarists over the years used a table, similar to that shown below, to understand the CO2 levels in their tanks.

KH,PH and CO2 relationship Chart

Please have a closer look at the table; you can see on the rightmost column there is KH and at the top row showing the pH value. The carbon dioxide level is demonstrated using three color bands; Green, Yellow, and blue.

You can use your aquarium water’s pH and KH values to determine the CO2 levels following this table. It should ideally be in the Green Zone.

This table can be ineffective if your water test kit is not accurate or not reliable. A slight measurement error can shift the results, meaning you may fail to get the exact CO2 level of your aquarium water.

Another drawback of using this table is, this table is prepared considering only the carbonic acid impacts pH. However, there can be nitric acid or some other organic matter which is well capable of altering pH.

Summary: Although the KH/pH/CO2 relationship is not the most accurate way to measure CO2 levels in your aquarium, you can quickly get a rough estimation.

#3: The drop checker turns yellow

Measuring CO2 level with Drop checker

The drop checker method is the most accurate way to date to understand whether your aquarium water contains too much CO2 or not. It uses a simple mechanism to yield a precise result.

It is a piece of equipment made of glass or plastic, filled with a solution of water and a pH reagent that sits inside aquarium water. The name of the pH reagent is bromothymol blue and is usually supplied with low-range pH test kits (6.0-7.6). 

Close look of CO2 drop checker
Close look of CO2 drop checker Owner: James Martin

CO2 in the aquarium water reacts with the solution and changes the color. Attach the drop checker with the solution and observe in a few hours. If your aquarium contains too much CO2, the solution color will change to yellow. If CO2 is ideal, it will show green, and for not enough CO2, it will remain blue as its original color. 

The solution inside the drop checker is kept separate from the tank water by an air gap. That’s why it takes a few hours for carbon dioxide to diffuse from the tank water to reach the solution and react. Although it’s highly accurate, you will not get the instant result; instead, you have to wait for hours. 

The best part is, it’s not expensive, very cheap, you can check its price here.

CO2 drop checker
CO2 drop checker Owner: Paul Johnson

Summary: A drop checker is the most reliable way to measure the CO2 level in your aquarium. Although it takes 2-3 hours to deliver the results, I recommend using it in your aquarium.

How to reduce CO2 in your aquarium (O3 Ways)

#1: Adjust needle valve

Adjusts the Needle Valve of CO2 injection

When you already know that the CO2 level in your aquarium is high, The first thing you should do is to cut the external source of CO2.

Operate the needle valve of your CO2 regulator to stop the CO2 injection. After that, follow the two methods below to reduce CO2 levels quickly.

Check the CO2 regulator if it is malfunctioning or providing erratic output. Some regulators also show ‘end-of-tank-dump’ syndrome, meaning injecting large amounts of gas right before the tank becomes empty.

If it is faulty, change the regulator to a reliable one. Suppose it’s ok; when the CO2 level drops to an acceptable range, adjust the needle valve to maintain a safe CO2 range. 

Summary: At a high CO2 level, immediately stop the CO2 injection, check for the faulty regulator, replace or rectify that, adjust the needle valve, and set it at a safe CO2 level.

#2: Do A Large water change

Large Water Change reduces CO2

The easiest and most effective way to instantly reduce the carbon dioxide levels in your aquarium water is to perform a large water change (50%). It is a quick fix to save your fish from carbon dioxide overdosing.

Use dechlorinated and the same temperature water as in the tank to avoid further stress to your fish.

I have a detailed article on how to change the tank water without killing fish. Please check that for further learning.  

Summary: 50% water change will immediately reduce the CO2 level to half of its original value; however, be careful not to shock your fish. 

#3: Increase Aeration of the tank

Aeration drives CO2 away

At the water surface, Oxygen and Carbon dioxide exchange occur, and it’s a continuous process. Your fish absorbs oxygen from tank water, and it converts into carbon dioxide in their bodies which they exhale.  

Carbon dioxide (CO2) releases from the water into the surrounding air, which is important; otherwise, your fish may die because of CO2 accumulation.

Increasing the aeration in the tank will expedite the process and drive down the CO2 concentration. So get into the action of reducing CO2 level with the help of an air pump and airstone. 

Summary: Aerating the tank will force CO2 to escape from aquarium water which will help to reduce CO2 concentration quickly.

#4: Extra Tips-Night is dark and full of terror! (be careful at night)

Be careful !Turn off CO2 at night)

When the sun is not out there in nature at night, plants respire in reverse, meaning they produce carbon dioxide and consume oxygen. The same thing happens to your aquarium because you cannot keep the light on all the time

You have to turn the light off at night to maintain the circadian rhythm of aquatic critters and prevent nuisance algae growth in your aquarium. Also, overexposure to light is detrimental to plant growth. 

When you switch off the light, plants start to use oxygen and deliver CO2 in return. So now you understand, the most possible time for your fish to feel carbon dioxide intoxication is at night.

Therefore, close your CO2 injection system immediately after the lights are off, keeping all other equipment, including the filter, and air pump, running.

Summary: At night, when lights are not on, you should also turn off the CO2 injection to prevent raising the Co2 level above the safe range. 

Final Thoughts

I hope you have a thorough understanding of the signs of high CO2 levels in aquariums and how to lower it if you unintentionally overdose.

If I summarise, you can primarily understand the signs of too much CO2 in aquariums by observing the fish’s behavior. After that, you can get a rough idea from the pH, KH, and CO2 relationship chart, and more accurately, through the color change in your drop checker.

I would like to hear about your experiences using CO2 in planted aquariums; please leave a comment below. If you find this guide effective, feel free to share it.

Sujit Modak

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