Why is carbon dioxide (CO2) the most important plant nutrient in the aquarium?

Carbon dioxide is the nutritional basis for aquatic plants which is supplemented through micro and macronutrients and light. The balance between all these elements is necessary for thriving plant growth in the aquarium. In photosynthesis light energy binds CO2 with the water and sugar is produced (nutrition for the plant). During this process oxygen (O2), which is required by other living organisms (bacteria, all animals), is released.

In the air the carbon dioxide content is currently constant at about 400 ppm (parts per million parts). Therefore it is always available in sufficient and – more importantly - in constant amounts for the land plants.

Under water this is totally different. Due to the partial pressure gradient to the atmosphere the carbon dioxide content in the water adjusts to about 2-4 mg/l when there is no additional carbon dioxide supply. This is not enough for the underwater plants to grow well.

This effect can frequently be observed in the wild. Tropical aquatic plants often form densely concentrated “colonies” at spots where water containing carbon dioxide seeps. As soon as the carbon dioxide in the water becomes too low, the aquatic plants withdraw from the waters again.

In aquariums without any additional carbon dioxide supply, there is always a lack of this important plant nutrient, since the recommended value for good plant growth lies between about 15 and 25 (up to 35) ml/g. This can only be compensated by supplying carbon dioxide gas with a CO2 system. For this purpose the CO2 plant care systems in the versions Bio CO2 or as pressurised gas cylinders variants Disposable (u) and Refillable (m) are very well suited.

As to the perfectly reasonable objections of some aquarium owners: “My plants display flourishing growth even without additional CO2 fertilisation.”

This may well be true, but you need to know the exact circumstances of the aquarium in case. In aquariums with relatively weak light conditions and with plants which tend to be slower growing, such as various Anubias, fern and Cryptocoryne species, the amount of CO2 produced by fish and bacteria, can definitely be sufficient for the survival and growth of the plants. But these cannot be described as having vigorous growth if you compare them with plants with an additional carbon dioxide supply.

Conclusion

Carbon dioxide (CO2) is the most important plant nutrient for aquatic plants in our aquariums. Yet the importance of additional CO2 dosing is underestimated by some aquarium owners. To counteract this lack carbon dioxide, which can be added through pressurised gas cylinders or bio-CO2, is required. Liquid CO2, as we will explain in a five-part blog, Is not available under normal conditions (temperature: 20 °C and atmospheric pressure: 1 bar or 1013 hPa).

© 18.06.2016
Matthias Wiesensee
Matthias Wiesensee
M.Sc. Wirtschaftsinformatik
JBL GmbH & Co. KG

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