Carbon dioxide is necessary for the development of aquarium plants but, at a certain level, it becomes toxic to fish. It is therefore important to know the correct concentration before adding CO2 to the aquarium.

An overdose of carbon dioxide is one of the main causes of fish death in planted aquariums. Your task is therefore to find the correct balance so your fish and plants can live happily and harmoniously.

The importance of CO2 in an aquarium

Carbon dioxide is required by plants for photosynthesis. Since CO2 is an essential part of the photosynthesis process, healthy plant growth depends on correct CO2 levels.

However, the CO2 requirements differ between plant species. Some aquatic plants can grow well without external CO2 addition in the aquarium because the aquarium water contains some CO2 produced by fish, bacteria and other aquarium inhabitants.

The carbon dioxide level in the air is about 400 ppm, so floating aquatic plants can easily get enough carbon dioxide for themselves. But in the water the CO2 falls at about 2 to 4 ppm which is not enough for most underwater plants. This is why CO2 supplementation is necessary and why it is a safe way to do so with a CO2 system.

What are the safe CO2 levels in aquariums?

In order not to harm all the inhabitants in a home aquarium you need to find enough CO2 so the fish and plants can thrive properly.

A safe level of carbon dioxide in an aquarium is less than 32 ppm. However, the CO2 toxicity threshold varies according to the oxygen level in the aquarium. In a poorly aerated aquarium, fish are more prone to CO2 poisoning. But if you can maintain proper dissolved oxygen levels in the aquarium, the safe limit of CO2 increases.

Summary: All aquarium fish and invertebrates show stress symptoms when CO2 levels reach or exceed 32 ppm. Try to keep CO2 levels in a safe range.

What are the signs of too much CO2 in the tanks?

Fish behavior

Your fish can signal high CO2 levels by changing their behavior:

• Fish become lethargic
• Lack of appetite
• Fish stay near the water surface and breathe heavily
• Jumpy movements
• Fish die

With elevated CO2 levels, the fish suffer from a lack of oxygen. They float to the surface and try to suck air with their mouth wide open. But do not confuse this behavior with labyrinthine fish, which occasionally float to the surface to take a sip of atmospheric air - this is their normal behavior.

If you observe all the fish at the water surface at the same time and breathing heavily, this is definitely a sign of a lack of oxygen and can also be a sign of high CO2 levels.

Summary: Observing fish behavior can be a great way to understand elevated CO2 levels, but it can be tricky if you are a novice aquarist.

How pH and KH relate to CO2 levels

Another common way to understand CO2 levels is to use the relationship between pH, KH and CO2. The pH scale determines how acidic or basic a solution is. Consider aquarium water. KH refers to the buffering capacity of the water, showing how sensitive it is to changes in pH.

When the carbon dioxide comes into contact with the water, it dissolves in the water and lowers its pH value. There is a fundamental relationship between CO2 and its effect on KH and pH.

Aquarists have been using a table similar to the one below for years to understand the CO2 levels in their aquariums.

The carbon dioxide levels are shown using three coloured bars: green, yellow and blue. You can use the pH-value and the KH-value of your aquarium water to determine the CO2 levels. Ideally, it should be in the green zone.

This table may be ineffective if your water test kit is inaccurate. A small measurement error can change the results, which means you will not get an accurate CO2 level in the aquarium water.

Another disadvantage of using this table is that this table is prepared taking into account only the effect of carbonic acid on pH. However, there may be nitric acid or some other organic substance that can change the pH.

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

Drop Checker turns yellow

The Drop Checker method is by far the most accurate way to tell if your aquarium water contains too much CO2 or not. It is a fairly simple method that gives accurate results.

The kit consists of a small dropper made of glass or plastic that is filled with aquarium water and a few drops of pH-reagent added to it. The PH reagent is called bromothymol blue and usually comes with low pH test kits (6.0-7.6).

The CO2 in the aquarium water reacts with the solution and changes colour. If there is an elevated concentration of CO2 in your aquarium, the colour of the solution will change to yellow. If the CO2 is perfect it will be green and if the CO2 is not enough it will remain blue.

The solution inside the drip trap is separated from the water in the aquarium by an air gap. This is why it takes several hours for the carbon dioxide to diffuse from the water in the aquarium to reach the solution and reactivate. The whole measurement process is quite long and can take several hours. It is a very inexpensive method to determine the CO2 concentration in the water.

Summary: The drip trap is the most reliable way to measure the CO2 level in your aquarium. Although it takes 2-3 hours to get results, we recommend using it in your aquarium.

How to reduce the CO2 concentration in your aquarium
Adjust your CO2 supply

When you already know that the CO2 level in your aquarium is high, the first thing you need to do is to turn off the external CO2 source. Then follow the two methods below to quickly lower the CO2 level.

Summary: Stop injecting CO2 immediately if CO2 levels are high

Large water changes

The easiest and most effective way to instantly reduce the carbon dioxide level in the aquarium water is to perform a large water change (50%). This is a quick solution that will keep aquarium fish from overdosing on carbon dioxide.

Use dechlorinated water of the same temperature as in the aquarium to avoid further stress in the fish.

Summary: A 50% water change will instantly cut the CO2 level in half from its original level, but be careful not to shock the fish.

Water aeration in the aquarium

There is a continual exchange of oxygen and carbon dioxide at the water surface. The fish absorb oxygen from the aquarium water and in their bodies it is converted into carbon dioxide which they exhale.

Increased aeration in the aquarium will speed up the process and reduce the CO2 concentration.

Summary: Aeration of the aquarium water will quickly reduce the CO2 concentration.