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 beginners in this hobby ask Natural rivers do not have CO2 injected for plants to grow, why do we need to do so in our planted tanks?'

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

Measuring CO2 levels

Carbon dioxide (CO2) saturation can be measured accurately in the water column through costly devices such as an Oxyguard CO2 analyzer. At the hobbyist level, cheaper tools such as drop checkers or CO2 titration test kits can give an estimation of CO2 levels within a range of error. CO2 is measured in ppm, short for Parts Per Million. Saying my aquarium has 1ppm CO2 means that if we took the weight of the water in the tank and divided it by 1 million, that is the amount of CO2 available by weight. The Oxyguard CO2 analyzer above gives a reading of 33ppm of 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 levels of carbon dioxide that range from 10 to 40+ ppm from organic decomposition and underground stores of CO2. In ground water, where off-gassing is constrained, CO2 often builds up to fairly high levels. This water reaches surface as freshwater springs.

At spring headwaters, the CO2 saturation can be as high as 40-50ppm, tapering off with increased distance. Plant growth is the most dense at the headwaters and wanes as the distance increases from the spring and CO2 levels subside. Many of the aquatic plant species in the trade come from areas with rich CO2 levels.

This is Mimulus in California. The aquatic plants here grow submerged permanently year round. The water measures around 30 ppm CO2, the source of CO2 being a volcanic spring about 3 miles up river. Photo credits: Tom barr

Rainbow Springs state park, Florida USA. CO2 levels in this spot is about 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 from Christel kasselmann's book, contains CO2 saturation data from various other rivers where commercial aquarium plant species are collected from. The variation is significant, from 10+ppm to 30+ppm. 

In nature, the level of CO2 may also not be consistent. For smaller water bodies, CO2 typically builds up throughout the night and is quickly utilized by aquatic plants once there is sunshine. In smaller ponds, CO2 levels and pH fluctuate a huge amount as the sun rises and photosynthesis starts - draining CO2 and causing an upwards rise in pH levels, often by a 1 full pH or more. Other water bodies that receive CO2 enriched spring water can have consistently elevated CO2 levels. Depending on the type of habitat that a certain species of aquarium plant is collected from, its CO2 demands can vary greatly.

Not all species have the same CO2 requirements. A species used to growing in CO2 enriched spring waters have very different requirements from species that are collected from rivers that have lower CO2 saturation.

Having elevated CO2 levels from CO2 injection allows us to grow a large variety of species with differing needs together. This CO2 injected tank has around 40ppm of CO2. 

Natural CO2 levels in aquariums 

We use an Oxyguard CO2 analyzer to test for CO2 levels. CO2 levels can differ greatly depending on the time of day the test is done. For planted aquariums CO2 build up in the night due to microbial and livestock respiration, and this CO2 is quickly taken up by plants when the lights turn on. This low tech tank measures 5ppm of CO2 just as the lights come on.

What about CO2 from the atmospheric, one might ask? Due to gas pressure laws, atmospheric CO2 only contributes a tiny amount of CO2 to water bodies after gas pressure laws are applied. A standing glass of water, in equilibrium with atmospheric CO2 levels, will only measure 0.6ppm of CO2. Having good gaseous exchange does little to improve CO2 levels in non CO2 injected aquariums.

A standing glass of water will alternate between a null (0) and 1ppm reading on the CO2 analyzer. Similarly, a tank with no microbial respiration, that is fully dependent on atmospheric CO2 will have hardly any dissolved CO2 in the water.

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

Many factors affects a tank's ability to generate CO2 - from the type of substrate used, to the maturity and nature of microbial mix in particular tank, to the flow pattern and off gassing rates for a particular setup. Not all substrates generate CO2 at the same rate; aquasoils do seem to generate more CO2 than inert substrates, but different types of aquasoil produces different outcomes as well. Due to the large number of confounding factors that can affect natural CO2 generation, natural CO2 generation in most aquariums is a wonky affair.

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

In this particular tank with a thick aquasoil base, we managed to measure 6ppm of CO2 before lights on. However, in similar tanks but with different brands of aquasoil, we have different measurements of 1-3ppm.

In this crowded shop tank we managed to measure 5ppm of CO2. The elevated levels may be partly due to the lack of plant mass to uptake the available CO2.

Conclusions

Natural CO2 generation in aquariums is generally inconsistent, which leads to a large variation of outcomes in non CO2 injected tanks. Being able to generate CO2 naturally is closely tied to success rates with 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 generate more CO2 naturally that do better.

Using aquasoil/soil substrates seem to contribute some CO2, but it is at a very low level compared to natural water bodies and the levels are not sustained throughout the day. As aquasoil/soil substrates deplete and age, this value changes over time as well. Most of the hobbyists and shop tanks that we measured show 1-2ppm of CO2 during the day, even with usage of aquasoil/soil substrates. 

This low level of CO2 generation allows the cultivation of many species, but seldom to their best form or density. CO2 injection raises CO2 saturation rates closer to levels found in natural water bodies where aquatic plant growth is abundant (between 10-40ppm). The greatest impact for aquarium plants tend to be the first 10ppm or so. While many easier species can survive at 1-2ppm of CO2, they flourish and show better coloration and density even at just 10ppm of consistent CO2.

This means that even a low level of CO2 injection has a big impact for most planted aquariums.

Plants grown in a CO2 injected tank have better coloration and density. Sufficient CO2 also allows plants to preserve their older leaves for longer. CO2 levels in this CO2 injected tank measures around 40ppm.

Impact of CO2 injection in tanks

CO2 injection dissolves CO2 gas directly into the tank's water and raises CO2 levels anywhere from 10ppm to upwards of 40ppm depending on how much is injected.

Many easy aquatic species do well just a a low level of 10-15ppm of CO2 while some picker species propagate much better at the 30-40ppm zone. With CO2 injection, it is also possible to sustain elevated CO2 levels throughout the light window, rather than just have a small spike of available CO2 that build up through in the night in non CO2 injected tanks.

CO2 levels in this tank are around 40ppm.

CO2 injection boost growth rates 5-10X times compared to growth in low tech aquariums and helps tremendously in growing carpets & colored plants. Plants have better form, color and health when grown in an environment where CO2 is optimally provided. Thin stems, stunted leaves, poor coloration, algae on plants are all common signs that CO2 levels are inadequate for optimal plant growth. Growth speed is not the only benefit that CO2 enrichment provides.

As carbon is the major component of plant tissue, it has the greatest impact by far on growth. Where we commonly think of NPK (Nitrogen, phosphorus, potassium) as important nutrients for plant growth, they make up a small portion 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 folks channel into thinking about fertilization - where actually the much larger impact factor is access to carbon. This concept applies even for 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 do 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. Fishes from stagnant pools with poor oxygen levels are much more tolerant of elevated CO2 levels compared to fish from fast flowing, oxygen rich rivers. In this particular tank, Cardinal tetras exhibit normal feeding behavior even at 70ppm of CO2. Discus are known to be more sensitive to CO2 levels, and dislike CO2 levels above 40ppm.

Most aquarium plants grow excellently at 30ppm of CO2, and most fish are comfortable at that level, so 30ppm is a good target zone 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 that supply aquatic plants for commercial trade will choose to grow aquatic plants in emersed form if possible. This negates the need for CO2 management, and many emersed forms are more hardy (emersed growth have to grow sturdier structures to hold their own weight against gravity, while submerged forms have water to support their weight). Emersed forms are thus easier to grow, and easier to transport. The picture above shows an aquatic plant nursery by Dennerle from Germany.