Why inject CO2 for a planted tank, isn't it unnatural ?

Why inject carbon dioxide? How do natural lakes/rivers work?

The question that many newcomers to the hobby ask is: Natural rivers do not have CO2 injected to help plants grow, so why do we need to do this in our planted tanks?

The truth is that the equilibrium levels of dissolved carbon dioxide (CO2) in our aquariums are quite low compared to the levels found in natural habitats. Most aquariums do not have a great capacity to generate CO2, whereas in some natural bodies of water - the decomposition of organic matter and natural CO2 from underground aquifers greatly increases CO2 levels. It is in these areas of nature where CO2 levels are elevated that we find rich aquatic plant growth.

Measuring CO2 levels

Carbon dioxide (CO2) saturation can be accurately measured in the water column using expensive equipment such as an Oxyguard CO2 Analyser. At the hobbyist level, cheaper tools such as drop checkers or CO2 titration test kits can give an estimate of CO2 levels within a margin of error. CO2 is measured in ppm, short for parts per million. To say that my aquarium has 1ppm CO2 means that if we take the weight of the water in the tank and divide it by 1 million, that is the amount of CO2 present by weight. The Oxyguard CO2 Analyser above gives a reading of 33ppm CO2 for the tank above.

More details on CO2 measurement can be found in this article.

CO2 levels in nature

Natural environments that support thriving plant growth typically have carbon dioxide levels of 10 to 40+ ppm from organic decomposition and underground CO2 stores. In groundwater, where gas emissions are limited, CO2 often accumulates to relatively high levels. This water reaches the surface as freshwater springs.

At the headwaters of springs, CO2 saturation can be as high as 40-50ppm, decreasing with distance. Plant growth is densest at the headwaters, decreasing as distance from the source increases and CO2 levels decrease. Many commercially available aquatic plant species originate from areas with high CO2 levels.

This is Mimulus in California. The aquatic plants here grow permanently submerged all year round. The water contains about 30 ppm CO2, the source of which is a volcanic spring about 3 miles upstream. Photo credits: Tom Barr

Rainbow Springs State Park, Florida, USA. CO2 levels at this site are around 18 ppm. Massive flow coming out of this spring. Photo credits: Tom Barr

Big Spring headwater in California, feeding into Hat Creek. 40 ppm of CO2 measured with a high concentration of aquatic plant species. Photo credits: Tom barr.

Giant springs, Montana. 25ppm of CO2 measured. Photo credits: Tom barr.

This table, reconstructed from data in Christel Kasselmann's book, gives CO2 saturation data from various other rivers from which commercial aquarium plant species are collected. The variation is significant, from 10+ppm to 30+ppm.

In nature, CO2 levels may also be inconsistent. In smaller bodies of water, CO2 tends to build up during the night and is quickly utilised by aquatic plants when the sun shines. In smaller ponds, CO2 levels and pH levels fluctuate greatly as the sun rises and photosynthesis begins - CO2 is released and pH levels rise, often by a full pH or more. Other water bodies that receive CO2-enriched spring water may have consistently elevated CO2 levels. Depending on the type of habitat from which a particular aquarium plant species is collected, its CO2 requirements can vary greatly.

Not all species have the same CO2 requirements. A species accustomed to growing in CO2 enriched spring water will have very different requirements from species collected from rivers with lower CO2 saturation.

Elevated CO2 levels from CO2 injection allow us to grow a wide variety of species with different requirements together. This CO2 injected tank has around 40ppm CO2.

Natural CO2 levels in aquariums 

We use an Oxyguard CO2 analyser to test CO2 levels. CO2 levels can vary greatly depending on the time of day you test. In planted aquariums, CO2 builds up during the night due to microbial and animal respiration, and this CO2 is quickly taken up by the plants when the lights come on. This low-tech aquarium measures 5ppm CO2 when the lights come on.

What about the CO2 that comes from the atmosphere, you might ask? Due to the laws of gas pressure, atmospheric CO2 only contributes a tiny amount of CO2 to water bodies after gas pressure laws have been applied. A standing glass of water in equilibrium with atmospheric CO2 levels will only measure 0.6ppm of CO2.Good gas exchange does little to improve CO2 levels in aquariums without CO2 injection.

A standing glass of water will alternate between a zero (0) and 1ppm reading on the CO2 Analyser. Similarly, a tank with no microbial respiration and completely dependent on atmospheric CO2 will have very little dissolved CO2 in the water.

In mature planted tanks with a soil substrate, microbial respiration and animal respiration can raise CO2 levels to between 2-6ppm. In well planted tanks, this CO2 is rapidly absorbed when the lights are switched on, the same tank can measure 0-1ppm CO2 after a few hours.

Many factors affect the ability of a tank to generate CO2 - from the type of substrate used, to the maturity and nature of the microbial mix in a particular tank, to the flow pattern and degassing rates for a particular set-up. Not all substrates produce CO2 at the same rate; aquasoils appear to produce more CO2 than inert substrates, but different types of aquasoil also produce different results. Due to the large number of confounding factors that can affect natural CO2 generation, natural CO2 generation in most aquariums is an uncertain proposition.

Gathered CO2 data from hobbyists and aquarium shops show levels ranging from 1 to 6 ppm, with many planted tanks showing levels closer to 1 to 2 ppm when the lights are on. Just before the lights are turned on, during the peak of the night cycle, the same tanks can measure between 3-6ppm CO2.

In this particular tank with a thick aquasoil base, we measured 6ppm of CO2 before switching on the lights. However, in similar tanks but with different brands of aquasoil, we have readings ranging from 1 to 3ppm.

In this overcrowded shop tank we were able to measure 5ppm CO2. The elevated levels may be partly due to the lack of plant mass to absorb the available CO2.

Conclusions

Natural CO2 production in aquariums is generally inconsistent, resulting in a wide range of results in non CO2-injected tanks. The ability to generate CO2 naturally is closely linked to the success rates of plants in non CO2-injected tanks. Contrary to popular belief, it is not that low-tech tanks can grow well without CO2 - it is low-tech setups that can produce more CO2 naturally that do better.

The use of aquasoil/soil substrates does seem to contribute some CO2, but it is at a very low level compared to natural water bodies and the levels are not maintained throughout the day. As Aquasoil/soil substrates are depleted and age, this will also change over time. Most of the hobby and shop aquariums we have measured show 1-2ppm CO2 during the day, even when using Aquasoil/soil substrates.

This low level of CO2 generation allows many species to be grown, but rarely to their best form or density. CO2 injection increases CO2 saturation rates closer to levels found in natural waters where aquatic plant growth is abundant (between 10-40ppm). The greatest effect on aquarium plants tends to be in the first 10ppm or so. While many simpler species can survive at 1-2ppm CO2, they will thrive and show better colouration and density at just 10ppm of constant CO2.

This means that even a low level of CO2 injection will have a big impact on most planted aquariums.

Plants grown in a CO2-injected tank have better colour and density. Sufficient CO2 also allows plants to retain older leaves for longer. CO2 levels in this CO2-injected tank are around 40ppm.

Impact of CO2 injection in tanks

CO2 injection dissolves CO2 gas directly into the aquarium water and raises CO2 levels from 10ppm to over 40ppm depending on how much is injected.

Many easy aquatic species will thrive at a low level of 10-15ppm CO2 while some picker species will thrive much better in the 30-40ppm range. With CO2 injection it is also possible to maintain elevated CO2 levels throughout the light window, rather than just having a small peak of available CO2 that builds up overnight in non CO2 injected tanks.

CO2 levels in this tank are around 40ppm.

CO2 injection increases growth rates by 5-10 times compared to low tech aquariums and helps tremendously in growing carpets and coloured plants. Plants have better shape, colour and health when grown in an environment where CO2 is optimally supplied. Thin stems, stunted leaves, poor colouration, algae on plants are all common signs that CO2 levels are inadequate for optimal plant growth. Growth rate is not the only benefit of CO2 enrichment.

As carbon is the major constituent of plant tissue, it is by far the most important factor affecting growth. While we commonly think of NPK (nitrogen, phosphorus, potassium) as important nutrients for plant growth, they make up a small proportion of plant mass compared to carbon. A plant uses 10 times more carbon by mass than all the other 'macro-nutrients' combined. Think about how much energy people put into thinking about fertiliser - when the much bigger factor is access to carbon. This concept also applies to non CO2 injected aquariums.

In a planted aquarium, injected carbon dioxide (CO2) boosts growth speed as well as quality, and has high impact regardless whether you use low or high lighting. Contrary to popular belief, one does not need to have strong lighting to benefit from CO2 injection.  

Certain species such as the red Eriocaulon, quinquangular and Centrolepis drummondiana 'Blood Vomit' are particularly demanding with regards to CO2 and will not grow at all in non CO2 injected aquariums. They grow much better at 30-40ppm CO2 than at lower levels.

How high can you go? CO2 tolerance is species dependent. Fish from stagnant pools with low oxygen levels are much more tolerant of elevated CO2 levels than fish from fast-flowing, oxygen-rich rivers. In this particular tank, Cardinal Tetras show normal feeding behaviour even at 70 ppm CO2. Discus are known to be more sensitive to CO2 levels and dislike CO2 levels above 40ppm.

Most aquarium plants will thrive at 30ppm CO2 and most fish will thrive at this level, so 30ppm is a good target for CO2 injected tanks.

For a more detailed guide on targeting CO2 levels, head here.

If CO2 is so important, do farms inject CO2?

Plant farms supplying aquatic plants for commercial trade will choose to grow aquatic plants in emersed form where possible. This eliminates the need for CO2 management and many emersed forms are more robust (emersed growth has to grow stronger structures to support its own weight against gravity, whereas submerged forms have water to support their weight). Emersed forms are therefore easier to grow and transport. The picture above shows a Dennerle aquatic plant nursery in Germany.