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• CO2 in Planted Aquariums - The Advantages, Disadvantages & What Else to Consider

13 May 2022

The addition of carbon dioxide (CO₂) in an aquarium can dramatically improve the health and vibrance of aquatic plants. CO₂ is one of the key inert gases found in Earth’s atmosphere, albeit at a very low concentration compared to Nitrogen and Oxygen. Life on Earth is only possible because of plant's ability to absorb CO₂ and produce oxygen (O₂).

Photosynthesis is the process of converting energy from CO₂ and water (H₂O) into O₂ and glucose (sugars, present as C₆H₁₂O₆). Plants then emit the oxygen and use the glucose as the building blocks for their growth. CO₂ levels can vary in an aquarium, depending on a few factors. Fish produce CO₂ as a byproduct of respiration - so the more fish in a tank, the higher the CO₂ produced. Surface agitation increases gaseous exchange, the process of oxygen entering and carbon dioxide exiting the water. If there is not enough surface movement, CO₂ remains in the water. The population of underwater plants will also impact the CO₂ levels. The more plants there are absorbing CO₂, the less CO₂ there will be dissolved in the water. One popular way to improve the growth of plants is to increase the concentration of carbon dioxide in the aquarium. This article outlines the different methods of introducing CO₂, and what to consider when using it.

Different methods of introducing CO₂ in to an aquarium

1 - Pressurised gas injection.

The most popular method of introducing CO₂ into the aquarium is using a pressurised bottle of carbon dioxide gas. The gas is injected under pressure through a tube and chopped into tiny bubbles through a ceramic disc. The bottles can either be disposable or refillable, and the gas output is controlled by a regulator screwed in to the bottle. The regulator is able to safely limit the output of gas to prevent overdosing. An inline bubble counter allows a visual clue of how much CO₂ is being added to the aquarium. There are resources available online to calculate how many bubbles per minute an aquarium would need to reach desired levels. Any gas that reaches the surface will not increase CO₂ levels, so it's important to maximise the absorption rate. Ceramic diffusers are used to decrease the size of the bubbles of gas underwater. The same volume of gas in many tiny bubbles has a much larger surface area compared to one large bubble. Increased surface area increases the rate of gas absorption when underwater. Replacement of gas bottles depends on the size of bottle and rate of injection.

2 - Biologically produced CO₂

A less popular but less costly option for injecting CO₂ are kits that can produce natural CO₂. To create natural CO₂, yeast and sugar are mixed in warm water and left to ferment. Yeast produces excess levels CO₂ when consuming glucose without needing much oxygen. The gas is then injected in a similar method to pressurised gas via a tube and diffuser. These kits usually last for between 20-40 days, until the yeast exhausts all the sugar in the mix.

3 - Chemically produced CO₂

CO₂ can also be produced by mixing citric acid and sodium bicarbonate in warm water. The chemical reaction produces sodium citrate, water and CO₂. The carbon dioxide, under pressure, is then fed in to the aquarium in via tubing and a diffuser. Results vary, but this method can generally provide CO₂ for 30-60 days.

4 - Liquid carbon sources

Another method of increasing available carbon in the aquarium is regularly dosing a liquid carbon source. They copy the benefits of carbon dioxide injection without the initial cost and the potential issues that can arise. NT Labs Liquid CO₂ Boost is an easy to use daily supplement to increase available carbon in the water to boost plant growth. It can be used in addition to CO₂ injection to increase CO₂ levels without risking a pH crash or can be used on its own as a carbon source. It is available in 100ml bottles and dosed via a built in pump at 1 pump per 25L/day.

Considerations when using CO₂ in an aquarium

Whilst increasing CO₂ levels in a planted aquarium will benefit plant growth, there are potential issues that can arise too.

1 - Monitoring CO₂ levels

While most CO₂ setups will include a bubble counter to give a visual clue to the amount of CO₂ being added to the aquarium, it is difficult to calculate CO₂ levels from counting bubbles. If not enough CO₂ is being added, the benefits will be negligible, and too much gas can cause a pH crash and kill livestock.

It’s therefore important to be able to finely adjust and measure CO₂ levels. Low tech colorimetric pH checkers that sit inside the aquarium are an easy way to determine CO₂ levels. Calculating CO₂ concentration via pH is easy as the two are in an inverse relationship. When CO₂ is added to water, some of the gas forms carbonic acid, which has a relative pH value of 4.18. Adding a highly acidic compound to water decreasing the pH value. So high CO₂ levels equals low pH levels. The pH checker changes colour depending on the pH value, usually being blue if the pH is high and yellow if it is low. With the inverse, CO₂ would therefore be blue if it is too low, and yellow if it is too high. The ideal CO₂/pH level in a checker is usually a solid green.

For more accurate and direct control, pH controllers with electronic probes can automatically adjust the amount of CO₂ to prevent overdosing. This is achieved with an electronic solenoid valve connected to the CO₂ regulator. When the pH value of the water becomes too low, the controller shuts off the gas supply to the aquarium, preventing a pH crash. When the pH naturally rises to a safe level (as the CO₂ dissipates or gets absorbed), the controller will then resume gas supply.

2 - Restricting supply of CO₂ at night

Photosynthesis is one chemical chain reaction that most people can remember (or wish to forget) from school biology. It is one of the fundamental processes on Earth that has allowed life to flourish as it gives the planet our oxygen supply. But whilst plants giveth during the day, they also taketh away at night! When sunlight is no longer available, plants can no longer absorb CO₂ and produce O₂, but biological processes must continue. During their ‘sleep’ cycle, plants actually absorb oxygen and emit CO₂, respiring like most organisms. If CO₂ is injected into an aquarium 24 hours a day, coupled with plants emitting CO₂ at night, levels can become dangerously high. CO₂ injection should be regulated so it is only injected during daylight hours. For pressurised systems, the easiest way to control this is via an electronic solenoid valve connected to a timer plug. A solenoid valve will automatically shut off supply of gas when power the power is switched off. Many CO₂ users will have their CO₂ on a timer which starts injection one hour before the light cycle, and shut off one hour before the lights turn off. This gives adequate time for the CO₂ levels to increase before plants begin to photosynthesise, and prevents over injecting near when the plants will no longer be able to absorb it. If restricting CO₂ injection, ensure the vessel is able to handle an increase in pressure. Pressurised CO₂ gas is fine as the bottle will be of a regulation allowing it to handle the pressure the gas is stored at (usually 50psi but bottles are rated for 100psi+ for safety). Homemade CO₂ injection using plastic bottles may swell/explode if the output is restricted. Alternatively, an air pump on a timer so it only comes on at night can assist with gaseous exchange, dissipating CO₂ at the surface to prevent overdosing.

3 - Use of air pumps

Air pumps are are fantastic for increasing water movement and gaseous exchange. The range of accessories can even be humorous and give an aquarium a fun focal point. When injecting CO₂, agitated surface movement is the opposite of what the aquarium needs. Too much surface movement will increase gaseous exchange, preventing CO₂ concentrations from reaching desirable levels. This would just be a waste of gas! Aquariums with sumps can also have this problem, with increased water turbulence leading to excess dissipation. Air pumps are only useful at night if the method of CO₂ injection cannot be regulated.

4 - Water flow in the aquarium

Another important factor to consider when using CO₂ is whether it is being evenly distributed through the aquarium. CO₂ is a gas, and gas that is injected into water will rise to the surface. This is why diffusers are used to reduce the size of the bubbles, increasing their surface area to increase the absorption rate before they reach the surface. If there is a lack of water flow, CO₂ levels will be higher at the waters surface than the bottom. Carpeting plants such as Glossostigma sp.  or Lileaopsis sp. that only grow well in high CO₂ conditions and inhabit the lowest levels of the aquarium will not get enough of the injected CO₂ unless there is adequate flow. A small wave maker, usually sold for marine aquariums, will do the job great!

5 - Cost of setups

Like any hobby, there are various levels of investment. The same is true for CO₂ setups. Whilst the large refillable gas bottles being regulated by a pH controller will be an expensive investment, CO₂ can also be done on a budget. Many hobbyists dabbling in planted aquariums and CO₂ will start with liquid carbon sources such as NT Labs Liquid CO₂ Boost. It is a cheap and easy way to give plants a carbon source without the outlay of equipment or any of the risks involved with pressurised bottles. For those looking for more automation or a constant supply, then the injection systems will be the preferred option. Compare prices of disposable bottles versus those that are refillable. Setups that have a more expensive outlay are usually the ones with reduced ongoing costs.

When weighing up all the considerations for adding CO₂ to a planted aquarium, there is no denying the benefits it has to plant growth and health. Most hobbyists would agree that the difference in their plants is like night and day when CO₂ is added.

Plants cannot grow on CO₂ alone, so other nutrient sources must also be considered. Regular use of a liquid macro and micro element supplements such as NT Labs Plant Boost is vital. Plant Boost replenishes those nutrients plants absorb from the water. Aquarium substrates that are designed to provide plants with root nutrients will also have a positive effect on plant health and growth. For established tanks where replacing substrate would be too difficult, root tablets can be added into existing substrate to enhance nutrient levels and only need replacing every few months. Lighting also plays a vital part of plant growth. The go to technology for aquarium lighting now is LED lights. A quality, multi spectrum LED that can provide natural daylight conditions and intensity will give plants the light they need for better growth. Check out the range of ONF smart aquarium lights.

To summarise, adding CO₂ to an aquarium can be broken down in to a simple pros and cons list:

Pros:

- Increased plant growth and vigour

- Range of methods to add to the aquarium, liquid sources versus gas injection.

- Automation (liquid sources could be used with an auto doser, gas controlled via solenoid   valve).

- Easy of use versus results - differences are obvious very quickly.

Cons:

- Cost of setup equipment (if using injection methods).

- Use of chemicals (if using liquid carbon sources).

- Potential danger of pressurised gas (mostly mitigated when using certified equipment).

- Must be monitored to prevent potential pH crashes and livestock suffocation.

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