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Determination of CO2 concentration in aquarium water

Carbon is the basic building element of aquatic plants. The main source of carbon is carbon dioxide (CO2). Therefore, it is important to know for aquarist how many CO2 is contained in the aquarium water and whether it is enough for aquatic plants or not. Concentration of CO2 in a tap water is approximately equal to the concentration of CO2 in the air - about 3 ppm. When plants placed into aquarium they begin to actively consume dissolved CO2. As a result CO2 concentration quickly drops. The less CO2 in the aquarium water the higher the value of pH. The water becomes less acidic and less suitable not only for plants but also for many fish species.

СО2 and carbonates (HCO3- и СО32-) – are the main regulation tools for water pH control


CO2 supply is necessary for planted aquarium to give adequate dose of CO2 for aquatic plants and to maintain optimal for aquatic plants pH 6-7 of the aquarium water. The control of CO2 supply to the aquarium can be made in the following ways: using AQUAYER test pH+KH or using long-term CO2 test AQUAYER Drop Checker.

pH, KH and CO2 concentration are interdependent parameters of water. That is why determination of CO2 concentration in the aquarium water can be performed by the use of AQUAYER test pH+KH. To do this, measure the pH and KH (carbonate hardness). Next, compare these values using a special table with pH/KH relation and estimate the concentration of CO2 in the aquarium. The optimal concentration of CO2 for planted aquarium is 15-30 ppm.

рН/КН ratio

This method takes just 5 minutes to find out the concentration of CO2 in the aquatium. But keep in mind that over time pH in the aquarium drops, which introduces an error in the determination of the concentration of CO2 by this method.

drop checker

More accurate but slower method is the determination of CO2 in the aquarium using AQUAYER Drop Checker - glass vessel with the AQUAYER Indicator. Depending on the concentration of CO2 in the aquarium AQUAYER Indicator changes color from blue to yellow or green. Green color corresponds to 10-30 ppm of CO2. Yellow - more than 30 ppm. Blue - less than 10 ppm.
The problem with this method is slowness. The method is based of slow gas exchange between the two vessels. Depending on the structure of glass or DIY long-term test the speed of response can be from 30 minutes to several hours. Therefore, if you see the green color it is not to mean that at this moment CO2 concentration is optimal. It was optimal for about an hour (or two) back to the observation. AQUAYER Drop Checker has optimal structure which provides the response speed of CO2 concentration changes not longer then 30 minutes.

Which method for CO2 determination is better? Taking to account that the optimal range of CO2 concentrations is quite wide (15-30 ppm), the accuracy of CO2 determination in the aquarium is not so important. More important is to maintain a stable concentration of CO2 in the aquarium and the accuracy of both methods for this purpose is fully sufficient. To determine the exact concentration aquarists can use a combination of these two methods.