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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 Earths 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.


Its 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 sleepcycle, 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:


- 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.



- 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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