CO2 In The Easy Aquascape

Pressurised CO2 systems are rarely a feature of the beginner aquascapers set-up. These systems are expensive, complex to set up correctly, and potentially deadly to fish if done incorrectly.
CO2, however, is absolutely vital to photosynthesis and therefore a planted aquarium. The focus of this article is how to maximise CO2 levels in the aquarium naturally to easily enable healthy plant growth.

CO2 In The Easy Aquascape

The three factors affecting plant growth:

It might seem odd to start an article on maximising natural CO2, by talking about the three factors affecting plant growth, but we increase CO2 by adjusting lighting as the two are closely linked. CO2 and lighting are two of the three main factors affecting plant growth, the other is nutrients, which we have covered in the plant fertilisation article. The chart below is copied from that article and shows how nutrient levels should be set to slightly exceed plant requirements to ensure they never go 'hungry'. What it also shows, and which we will focus on in this article, is that lighting in the easy aquascape can easily exceed plant requirements and CO2 tends to be a limiting factor for plant growth. This creates a problem for us to overcome as this imbalance will ultimately lead to unsightly algae growth in the aquarium. It is fair to say that all aquaria are populated with algae, our job is to ensure that plants are growing as healthily as possible so that the algae is outcompeted for resources and does not have a chance to grow uncontrollably to nuisance levels.

Balancing CO2, light and nutrients for optimal growth in the non-CO2 easy aquascape.
For more detail on the plant nutrient requirements see this page.

The link between CO2 and lighting:

When the aquarium lights come on in the morning, the plants begin to photosynthesise. This is a process which converts CO2 to O2, just like we all learnt at school for terrestrial plants. The process underwater is remarkably unchanged, except of course plants struggle to access CO2 which is now dissolved in water. They are just not set up to do this effectively. This is also why CO2 levels in the planted aquarium can be increased so dramatically, to levels that kill off our fish, yet the plants will thrive. As we would also all have learnt in school, plants produce oxygen and animals produce carbon dioxide, and so the system is balanced providing that populations of the different species are in balance. What we don't always learn in school is that whilst animals produce CO2 at a fairly steady rate through the day and night, plants require light for photosynthesis and so, importantly to us here, they use CO2 only when our aquarium lights are on.

This also means that plants convert carbon dioxide to oxygen at a rate which is faster than our fish produce it as they have a shorter window in which to be able to do so. So in a properly aquascaped tank which holds many plants, we exacerbate the problem of providing enough CO2 as we are effectively overstocked with plants and so the ecosystem is not in balance. This is why you will often see it recommended to have a lighting regime of less than 8 hours, some even recommend 6 hours, as this is the length of time for which CO2 is available, having built up overnight. So we have built a situation whereby lighting is in excess, and CO2 is in scarce supply, so to address this we would surely need to reduce lighting... in fact, no there is a far better way. A siesta lighting regime...

Introducing siesta lighting:

Siesta lighting means having a break in the lighting period in the middle of the day to allow CO2 levels to naturally build again in the aquarium through the respiration of the fish as plants are no longer using it faster than it is being produced. We have included a graph below to try and explain this... The shaded areas are the CO2 levels in the water, the green represents an aquarium with a siesta lighting schedule, and the blue represents a normal lighting schedule where lights simply come on in the morning and off in the evening. What this shows, is that under a normal lighting regime CO2 levels in the aquarium are exhausted towards the end of the lighting period as we discussed above. This  not only means that plants are not able to photosynthesise and grow, it generates a dangerous imbalance with excess nutrients and lighting for part of the day - the perfect conditions for algae to grow. The horizontal lines on the graph show the average level of CO2 during the lighting period. You will notice that the average CO2 level under the normal lighting regime is lower than under the siesta lighting regime.

The advantages of siesta lighting for CO2 levels in the Easy Aquascape

If we focus in on the siesta part of the graph represented in green we can see that overnight when the lights are off, the CO2 level builds just like in the blue graph, and again they reduce from 08:00 for the first 4 hours of the lighting being turned on. However, rather than the CO2 levels continuing to fall after 12:00, they rise again whilst the lights are off. By 16:00 when they are set to turn back on, CO2 levels have partially recovered meaning that there is still a good concentration of CO2 in the water ready for the last part of the lighting regime. This means that with siesta lighting there is always enough CO2 available for healthy plant growth throughout the day, and that on average it is available in higher concentrations than under a normal lighting regime. Further to this with a lower overall lighting duration, the opportunity for excess light to be present is reduced. We now have a far more balanced, effective system, without the need for additional equipment to be installed.

The advantages of siesta lighting for plant growth should now be clear, however there are numerous side benefits as well. Reduced electricity use and increased viewing time for the aquarium to name just two... Under a normal lighting regime lights are generally left on for 10 hours, providing an opportunity after working hours to view the aquascape and feed the fish. Under a siesta lighting regime, the overall number of hours the lights are run is reduced, and so therefore is energy use. The lights are also on more in the evening enabling the aquascaper to enjoy the fruits of their labour far more.


To see the impact of CO2 on plant growth, cut a stem from a stem plant in your aquarium and remove it from the water, shake off any excess water droplets and expose this stem to airborne CO2 for a couple of minutes before returning the stem to the aquarium. Watch as there is an explosion of microbubbles erupting from this section (just like in the photo at the top of this page). You may also notice this phenomenon afer conducting a big water change. Known as "pearling", this effect will only normally be seen in high tech systems with high levels of pressurised CO2.

Should I use liquid CO2?:

A commonly offered alternative to a pressurised CO2 system is 'liquid CO2'. Please do not confuse this product as genuine liquid CO2. Without getting technical, this product is normally a fairly dangerous substance called Gutaraldehyde, widely used to sterilize hospital equipment! It is still dangerous when overdosed into the aquarium acting as a neurotoxin, and some species are more susceptible to it than others. Matt recommends only ever dosing half the recommended amount to be on the safe side. It provides a source of carbon to the aquarium which some plants can benefit from, though it is at least four to five times less efficient for them to use than true dissolved gaseous CO2. In fact, the jury is out within the scientific community as to whether these products actually help or not. Some plants, like Vallis may even suffer when such products are used. Ultimately whether or not to use this product is up to you, it can act as a algaecide which some find beneficial also.

Siesta lighting... maximises CO2 availability in the Easy Aquascape, minimises energy use, and maximises the opportunity to view the aquarium around your busy schedule - a win-win situation. Give it a go and see the benefits for yourself. Let us know if you have any questions, or how your plant growth has improved in the comments section below.

      Matt, Founder of ScapeEasy