CO² – Understanding CO² for Planted Aquariums

The majority of aquarists tend to shy away from any article that smacks of chemistry or science — and this is no less so when it comes to understanding why we need CO2 to have thriving planted tanks. But understand it, we must, or we risk losing all the time, effort and money we put into our carefully, lovingly aquascaped tanks.

So let me start with this quote from the famed aquarium artist, Takashi Amano, who said:

“We have to remember that we either live in nature, or not at all. Through building and maintaining beautiful natural aquaria, people relearn the intricate connections between forms of life: plants, fish, micro-organisms and humans. Riches and beauty come from harmony, from balance. Aquaria are great teachers of this truth.”

You may well want to want to ask, what on earth does this have to do with CO², or carbon dioxide? Well, it has everything to do with creating, or at least mimicking Nature in our planted aquaria.

So let me promise to make this as easy as is possible, even for those of you who hate chemistry, or science.

Why CO²?

The reality is that all life on earth is based on one fundamental element: Carbon. Without carbon, we would not exist — and neither would plants. The difference is that unlike us, plants have no way of getting to their food sources. They have to obtain their nutrients from their surrounding environment.

Plants use many macro and micro-nutrients, but carbon dioxide (CO²) is beyond any doubt the most important of all plant macro-nutrients essential to their survival. Without sufficient CO2, plants cannot photosynthesize, or convert light into energy-rich sugars, starches and all the other carbon-containing molecules that constitute a plant, nor can they produce new leaves and roots.

In nature this is no problem, but in the closed system of an aquarium we are faced with several factors we need to overcome to be successful. These are: Light, CO², micro-nutrients (or trace elements), and macro-nutrients.

We usually manage to supply an adequate amount of micro and macro-nutrients via fish waste and the addition of fertilizers. This leaves us with solving the problems of light and CO².

Plants use the process of photosynthesis to produce the carbohydrates they need for life. But photosynthesis requires light for energy — and CO2 to drive the chemical reactions. The process of photosynthesis also requires a specific threshold in light energy to start the process of photosynthesis. In other words, the light we supply must reach aspecific point of intensity, because if the light in our tanks is not bright enough, photosynthesis will not occur.

If our lights provide an intensity beyond that point (of bright enough) to some higher light level, photosynthesis will become progressively faster. Theoretically, this should mean that plants will grow and flourish faster. And yes they would — however, at this point, for most aquariums, supplementary CO² is required to keep on driving the process.

Paradoxical as it may sound, this is also true for the opposite : If you do not have enough bright light, you need to supplement with  CO2, if you hope to promote growth. I will explain this a little later.

So, yes, you can grow aquatic plants without CO² — but only up to a point. You will never attain the vigorous plant growth and luxurious foliage that is characteristic of a  CO²  supplemented tank — or the added benefit of little or no algae — without CO² dosing.

Understand that there is always CO² in our planted aquariums — even without us adding it by mechanical means. Fish ‘exhale’ CO² from their gills. Also, when we aerate our tanks (which you should not be doing in a planted tank!), CO², as well as oxygen is derived from the atmosphere and dissolved in the water because of a natural process called atmospheric equilibrium.

In nature, however, CO² levels are usually much higher than can be explained by animal respiration or atmospheric equilibrium alone — and true aquatic plants have evolved to this higher concentration of dissolved CO² in water. Unlike terrestrial plants, which absorb CO2 from the air through their stomata, aquatic plants take up CO2 directly from the water by passive diffusion. In fact, their evolution as water inhabitants delivered them with epidermis cells that contain chloroplasts, since this reduces the distance from their CO2 source to their photosynthesizing cells and their energy processes. The reason for this is that  groundwater rich with Carbon dioxide often feeds the streams and water habitats of true aquatic plants, and in these habitats natural concentrations of CO² up to several hundred times atmospheric equilibrium are common.

In general, aquatic plants therefore not only like, but actually prefer an environment with approximately a concentration of 10-15ppm (part per million) of dissolved CO².  In contrast, in atmospheric equilibrium, the CO2 levels are generally no higher than 2-3ppm. So, as you can logically conclude from this, CO2 injection is essential for vigorous aquatic plant growth, and even more so with higher light levels!

But what does this mean for the fish in our planted tanks?

Since our fish depend on a CO² concentration differential between the levels in their blood and the levels in the water to transfer gases via expiration through their gills, high levels in the water will reduce the amount of CO² that can be transferred.  So, theoretically, high concentrations of CO² in the water could  block the CO² respiration of fish and therefore could cause oxygen starvation. But this is unlikely to ever be a problem if your plants are flourishing and your  CO² is dosed properly. It is proven fact that a concentration below 30ppm is safe for fish. (The recommended rate is 15–30 mg/L.)

Also, there is a common misconception that water can hold only so much dissolved gas, and that adding CO2 will displace the oxygen in the water. This is not true. In fact, if enough CO2 and light is present to enable vigorous photosynthesis, oxygen levels in the water  can reach 120% of saturation. This is even so at night, when the plants stop using CO2 and start using oxygen: the oxygen levels will stay about the same as in a typical non-planted aquarium.

A word of important advice here: If you have any air pump in your planted tank, switch it off! In fact, retire your air pump entirely.  It cannot be emphasized enough that an air pump should never be part of a planted aquarium. The function of an air pump is to supply oxygen (O²) to fish and/or invertebrates in aquaria that have no sustainable oxygen production from aquatic plants. In all planted aquaria, there should be more than sufficient oxygen for both fish and invertebrates, even at night when there is no photosynthesis. When plants, fish and invertebrates respire during the night, CO² is produced and dissolves readily in the water. This CO² is then used in photosynthesis by the plants once the light is switched on the next morning. If you have a running air pump, the CO² is de-gassed to the air  — in the same way carbonic acid is lost from a soda or beer can when shaken. Now, that is really counter-productive, considering we are trying to get CO² into the aquarium, not out!

The relationship between light and CO²

The relationship between light and CO² levels is important. If an aquarium has low light and low CO², there is very little energy to play around with for the fluctuating up and down processes nature uses to regulate the pools of chlorophyll or enzymes contained in a plant. So when we add a little CO² to the low light system, the plant needs less energy and resources to uptake CO², leaving it more energy to optimise its use of the available light for its photosynthesis. Thus it can produce chlorophyll without any fatal consequences for its energy. This means that even though we have not raised the light intensity, the plant can now use the available light more efficiently.

We can use exactly the same explanation to understand why an increased light intensity can stimulate growth even at very low CO2 concentrations. Because there is more light available, the plant needs to spend less energy on using the light to photosynthesise, leaving the freed-up energy to more efficiently extract the CO² available in the water.

In Nature there is always a constant play between using what is available and the eternal choice between processes, and to to what extent an organism’s life energy is expanded to which process, in order to secure its survival. Life wants to live. This is no different in the case of aquatic plants.

The logical conclusion that must follow the two explanations above, is that an increase in lighting and the addition of a CO² infusion should lead to a flourishing planted tank. This is because all the energy freed-up in this case can now be invested in growth.

While we can easily provide macro and micro-nutrients to our plants with commercially available fertilizers, it is often a more difficult and expensive task to provide adequate light over the planted aquarium.

Provided they are meant for plants, both fluorescent light and halide lamps in sufficient numbers will produce a sufficiently high light intensity, especially if they are also rigged up with effective reflectors; but in deep aquaria, meaning more than 45—50cm deep, it is very difficult to offer enough light to small light- demanding foreground plants. The preferred first remedy for this is always CO² addition, before any other action is taken! The reason is that even at very modest light intensities the addition of CO² can very quickly bring about a significantly obvious change in the performance of the plants in your aquarium. The exact CO2 dosage may vary from tank to tank, but concentrations from 10-15ppm will already vastly improve plant growth. The proof lies in the pudding: Use CO² and you should see your plants flourish — even plants that were barely able to survive before!

In fact, use COand within a short while your main activity will be vigorously pruning, growing and exchanging plants with other planted tank enthusiasts!

So you see, there is no secret to successfully growing a planted aquarium. Nor do you need science to understand the techniques. Those gorgeous planted tanks shown in pictures have not come about by luck, green fingers or any magic. They all flourish because of CO² injection!

So how are you going to go about adding  CO² to your aquarium? There are two possibilities for attaining a CO² dispenser. You can buy a system — streamlined, functional, and coming with all the bells and whistles, but pricey; or you can build your own low-cost system, and accept the bit of hassle that goes with it. You will find diy instructions on the internet, but also check forums to find out what worked for other aquarists. Many of the forum participants went on to improve previously given diy designs. One of the best , and very well illustrated is “A Treatise on DIY CO2 Systems for Freshwater-Planted Aquaria” by John LeVasseur . You will find it via Google.

Back to the top