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The best CO2 levels for an aquarium

Carbon dioxide (CO2) is the single most vital nutrient for plant growth. In fact, it constitutes 50% of a plant mass. Therefore, a low level of CO2 in your aquarium water will reduce the plant growth in it. The benefits of adding carbon dioxide in planted aquarium has become very popular among aquarists. CO2art is dedicated to providing their customers with convenient kits as well as to meet the rising demand. 

However, injecting the gas into the tank is not a simple case where you just add as much as possible. This gas is too much deadly to your fish when overdosed. You should thus monitor or test the level of gas levels of carbon dioxide in your tank using a CO2 Regulator. This will help you to fertilize your plants adequately and not to endanger your fish. 


Sources of CO2 in planted aquariums

  • The surrounding air through gas exchange at the water surface 
  • Respiration by the life in the water such as fish as well as the aquatic plants when not lit.
  • Deliberate CO2 injection to encourage plant growth


The best CO2 levels in planted aquariums


Fish do not flourish in high levels of CO2. Concentrations of CO2 above 30-35mg/l are indeed toxic to most aquarium lives. By ensuring proper setting of your aquarium, you reduce the problem of excess CO2. Such a problem can only occur in a good set-up tank if the gas fertilization results to an overdose. 

However, the gas may build up to a dangerous CO2 levels in an over-stocked tank with poor water circulation (so poor gas exchange), even without the addition of extra CO2. This mostly happens at night when the oxygen use is high along with more released CO2; watch out when you see fish gasping at the surface.

Therefore, you can only keep both fish and aquarium plants happy in the same aquarium by balancing the carbon dioxide necessities of the plants with the maximums of fish. By managing the concentration of CO2, you ensure a healthy ecosystem in your planted aquariums.


Determining the CO2 level

You can measure the amount of carbon dioxide in your planted aquarium water by testing both the KH (carbonate hardness) levels and the pH with the test kits. You can then use the results of this basic relationship to evaluate the concentration of the gas. You need to get the standard reference table and then cross-reference the two values, and their intersection gives the CO2 levels in your tank in mg/liter. 

The best safe levels of the gas for plant growth are between 15 mg/l and 35mg/l. If your tank uses mature or peat substrate with a lot of debris and organic matter, then this method is not accurate. 

 The drop checker method

Aquarists also use other CO2 regulators such as drop checkers and CO2 constancy devices to monitor regularly carbon dioxide levels. For this case, you measure pH fluctuations by employing a calibrated tracking fluid that works by changing color when the levels of CO2 changes. Since you install drop checkers permanently in your planted tanks, you will get an at-a- glance reading of CO2. You will see a dark blue color if there is too little, a green color if the CO2 level is optimal and a lime green to yellow color if there is excess CO2. Some people sat that lime green color of drop checkers is optimal but remember that there are species extremely sensitive to high CO2 levels in aquarium water. For sure this method it's easier because we don't have to analyze the whole water chemistry to safely monitor the CO2 levels.

DYI liquid ph test kits

And here only a few words... Athough there are plenty of DYI recepies how to make a drop checker on Internet forums, be aware, that mixing a well calibrated fluid is not an easy task. We encourage you to choose reliable brands when choosing a drop checker. One advantage of this solution is that companies are specialized in producing such a ph reagent and will probably do it more accurately. The second is that the glass provided has better design than a vodka glass. And the third that the price is reasonable and you can also purchase the solution separately. All three combined together win over unaccurate home ph reagent.

Try this method with CO2Art!

We find this method very effective because it allows the aquariust regularly monitor ph levels which is essencial for fish. And here, also, remember that there are fish species which live in nature in water where ph levels are higher because of e.g. rocks in the substrate (e.g. Malawi Biotope). Always read as much as you can before providing new species to your aquarium becuase it may turn out, that they have special requirements.

If all the information considered, you can start with installing our Drop Checker Kit. Take a look at our quick tutorial how to install a drop checker in your aquarium.


Read more about this effective method in our detailed article about drop checkers.


As you can see, using carbon dioxide in planted aquarium is not complicated in times of Internet, access to proper tables with ready numbers and necessary equipment. But you need to remember that this gas can be lethal to aquarium fish if overdosed so monitoring is essencial. For best developement of your underwater ecosystem, we encourage you to rely on trustworthy brands. Let us help you to have CO2 in your aquarium and safely control CO2 levels accurate for your aquatic plants and aquarium lives. You'll see gas fertilization results in the blink of an eye. Rapid and healthy growth of your plants will be observed at first. Trust our convenient kits and your planted tank will never be the same aquarium.