Carbon Dioxide Injection

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Carbon dioxide (CO2) injection is a valuable aspect of maintaining a thriving planted aquarium, and it is also the most precise method for decreasing pH in aquariums. Because carbon dioxide dissolves in water to form carbonic acid, the relationship between CO2 and pH in water is tightly connected, and impacts the health and growth of aquatic plants.

The pH of the aquarium plays a crucial role in plant health and growth. Maintaining a slightly acidic pH around 6.0-6.5 is desirable, as it allows plants to access essential nutrients efficiently, leading to robust growth and vibrant colors. Achieving this optimal pH range requires regular monitoring of alkalinity or carbonate hardness (KH), as it acts as a buffer to stabilize pH and prevent rapid fluctuations. Striking the right balance with CO2 injection is essential, as excess CO2 can stress fish and cause pH fluctuations, and in excess can certainly cause harm to your fish. Achieving optimal CO2 levels ensures lush plant growth and a vibrant aquascape.

Monitoring CO2 Levels

The first step is determining your current carbon dioxide level in your aquarium, as accurately with any chemical dosing regimen, quantifying your current levels is obligatory. Some hobbyists choose to use a drop checker for economical carbon dioxide monitoring; however, these methods are a bit lacking in precision and can be time-consuming.

As an alternative, monitoring CO2 by using your pH and alkalinity as a vector can be a quick and easy way to determine your carbon dioxide concentration.

This method utilizes the carbonate hardness (KH is a good approximation of total alkalinity for this method) and pH values to estimate the CO2 concentration, as carbon dioxide concentration bears a mathematical relationship with these two values.

The carbon dioxide concentration of your aquarium can be approximated by the following equation:

[CO2] = 12.839 * dKH * 10(6.37 – pH)

You can literally plug that formula into an excel spreadsheet to calculate carbon dioxide concentration on your own.

Of course, I’m not actually suggesting that your average hobbyist plug these values into a calculator each time they’re looking to adjust their carbon dioxide levels or decrease their pH. However, these values have long been presented in the form of a chart or table of carbon dioxide values that are easy to consult when trying to pinpoint your aquariums CO2 concentration such as the table below.

If you consult the chart above you can see that the amount of CO2 required to get your pH down to a suitable range for plants increases exponentially as the water increases in hardness. For example, if your carbonate hardness is only 1dKH, then 30 parts per million (ppm) of carbon dioxide dissolved is all that is necessary down to a pH of 6.0, whereas if your KH is at 10 dKH you will need over 300 ppm CO2 in order to get your pH down to the desired level.

The appropriate value for carbon dioxide varies depending on situation. However, you will want to maintain values greater than 20 ppm for best results, and limit your dosing to about 30 ppm. I highlighted values that I consider to be acceptable (between 15ppm and 30 ppm.) You’ll also notice that the larger values on this chart are highlighted with a pink color, and lower values in yellow.

Excessive CO2 can lead to stress fish and other organisms in the aquarium, and can be potentially fatal to those organisms. The red-highlighted values indicate carbon dioxide concentrations that may be dangerous for animals like fish or shrimp living in the aquarium. These values are approximate; however, I’d urge you to use caution when approaching the upper limit of the acceptable range for CO2 in the aquarium. Maintaining precise control over CO2 levels is necessary to create a stable and thriving aquatic environment.

It is important to note that this table can only provide an approximation of the carbon dioxide concentration in your aquarium. The pH of the aquarium is influenced by a variety of substances such as nitric acids, sulfates, phosphates, humic acids, and other organic matter. As a result, pH readings may frequently be lower than what standard tables assume, possibly resulting in inaccurate CO2 calculations even with precise measurements of the water’s KH and pH.

Regulating CO2 Injection

Achieving the desired pH range and CO2 concentration requires careful consideration of various factors. Monitoring and adjusting CO2 levels based on plant growth, lighting intensity, and tank size are essential for maintaining the right balance.

My recommendation is to start by establishing what pH you’d like your aquarium to be long-term, and consult the table for the appropriate carbon dioxide concentration you’re looking to achieve. Using the chart above you can determine what your carbonate hardness will need to be, and you can work from there.

For example, let’s say we’re working with a standard aquascape where the ideal pH is something like 6.5 and the ideal carbon dioxide concentration is around 30ppm. Using the chart above you can see that we’ll want to have a carbonate hardness around 4dKH, maybe even a touch less.

The old-school method for establishing the proper carbonate hardness is to mix your RO water with your tap water, presuming your tap water has some not insignificant level of carbonate hardness itself (a fairly safe assumption.) Thus, if your goal is 4dKH and your tap water is running 8 dKH, mixing your water 50/50 with reverse osmosis filtered water will provide you that goal KH value. If your tap water tests at 12 dKH, you know that you need to mix your water at a closer to 30% tap water with the rest as RO.

Another method is to add dry chemicals to reconstitute your water, which was stripped of minerals and buffering capacity in reverse osmosis filtration. Numerous products exist for this purpose, and all are some mixture of minerals and salts. Always follow the directions carefully. It is very prudent to mix your chemicals in a bucket or jug, and testing the alkalinity value of the water prior to adding it to your display tank.

Testing after you mix and adjusting to suit, you can establish a recipe using either of these two methods fairly quickly.

Regular testing of alkalinity or carbonate hardness (KH) helps aquarists gauge the buffering capacity of the water, ensuring a stable pH level. It is important to note that the biological cycle in your aquarium will be producing acids, and the result is a KH that, “like the sands through the hourglass,” are always decreasing. In a closed system it is only a matter of time before alkalinity is diminished, the pH drops like a rock, and the result is a crashed system.

Thus, it is vital that you continue to monitor your KH level while injecting CO2 to ensure that you are not over-dosing CO2 and potentially endangering your aquarium inhabitants.

CO2 Source

The easiest way to provide CO2 from your aquarium is to deliver it via a compressed metal canister, much like a scuba tank. You can procure these tanks from a local gas supplier; these locations will often refill the tanks as well.

In addition to the pressurized tank, you will need some kind of regulator valve that you can use to adjust the gas exiting the tank, as well as to cut carbon dioxide gas completely in the case of an oversaturation of CO2 or just at the end of the day’s photoperiod. If your regulator doesn’t have an electronic shutoff, a solenoid valve is a great addition to this setup as you can employ a simple timer to turn off your CO2 or regulate delivery with a CO2 controller.

Carbon dioxide can permeate standard silicone airline tubing, so you will also want to procure some CO2-proof tubing to deliver the gas from the regulator to a bubble counter or delivery device to ensure that every last bit of the carbon dioxide from your tank makes it into your aquarium.

There are DIY methods to produce CO2 as well. A yeast chamber can produce a reliable quantity of CO2 each day, though simply not to the scale and ease of regulation of a compressed gas tank. You can make your own yeast chamber, using a plastic soda bottle. The bottle itself needs to be filled with some dechlorinated freshwater, a starter dry yeast addition, and some white sugar. The yeast will consume the sugar and produce carbon dioxide as they grow. You can cap the bottle and pipe the gases created into a simple carbon reactor setup and viola, DIY carbon dioxide. There are a few caveats of course. The bottle has a distinct yeasty smell that some people find unpleasant. Additionally, carbon dioxide gas isn’t the only byproduct to the yeast’s glucose metabolism as they also produce small amounts of alcohol.

Maintenance of your yeast reactor is simple. You need to feed the yeast on a regular basis, a small addition of sugar on a daily basis is usually recommended. In addition, you’ll want to change the water in the yeast reactor on a regular basis to prevent the buildup of alcohol, which can have a detrimental impact on your CO2 production.

CO2 Delivery

Proper CO2 diffusion methods, such as using diffusers or reactors, play a significant role in distributing CO2 evenly throughout the tank.

In smaller setups, a simple diffuser is probably your best bet. They can be attached to the side of the aquarium and connected to your CO2 tank regulator. As you upsize the system, it becomes more common to filter the tank with a cannister filter located underneath the display. Once you have that simple bit of plumbing established, it’s not a far leap to start placing devices in the path of the flow to locate equipment out of sight instead of within the display.

A carbon dioxide reactor is a small vessel that increases the contact time with the CO2 gas you’re injecting with the water, allowing you to inject CO2 into the water without the unsightly diffuser in your display, they also help my evenly distributing carbon dioxide in the aquarium promoting stability of the system.

For larger systems, particularly systems already employing a sump-style filtration system, simply injecting CO2 into a venturi is a great option for CO2 delivery.

For both a CO2 reactor and a venturi system it may be necessary to shunt some of your system flow through the device if the CO2 or reactor cannot handle the entire flowrate of the pump. In larger systems it may be prudent to add small recirculation pumps to the display to prevent dead spots in the tank, as effective circulation prevents localized pH changes and promote uniform plant growth.

CO2 Monitoring

In addition to monitoring your pH and alkalinity, to approximate your CO2, it is essential you continuously monitor how much carbon dioxide you’re using in your delivery method. This will allow you to make minor and measured adjustments to the flow through your reactor for example, or the amount of CO2 you’re delivering to your diffuser, and will help eliminate the guesswork as you adjust your CO2 delivery rate.

First, a bubble counter is a simple way to quantify how much CO2 you’re injecting, and many regulators come with integrated counters. The counter itself is a simple device, a small chamber filled with water or preferably glycerin for the increased viscosity. By adding CO2 into the bottom of the device you can see bubbles rising through the liquid inside the counter. You can then quantify your CO2 injection in terms of bubbles per minute, a common unit of CO2 injection measurement.

Dealing With Dark

It is highly advantageous to turn your carbon dioxide delivery method off when the lights to your aquarium aren’t on. The reasoning behind this is that your tank will achieve a balance during the day between the oxygen produced via photosynthesis with the carbon dioxide your tank inhabitants produce via respiration, as well as the CO2 you inject into your aquarium water.

During the day, photosynthesis by aquatic plants releases oxygen and stabilizes pH, while at night, respiration processes by plants and animals lead to an increase in CO2 and a decrease in oxygen, potentially causing pH fluctuations. Not only is it vital to mitigate large pH fluctuations, but over-saturating the water with CO2 is potentially deadly for the aquarium inhabitants.

Diurnal CO2 injection regulation can be as simple as installing a timer to shut off a solenoid valve. But there are other things you may want to consider, including implementing reverse-photoperiod aeration or utilizing a pH controller for precise CO2 dosing. By understanding and managing these factors, aquarists can maintain a healthy and stable environment for their aquatic inhabitants.

Mastering the art of managing CO2 levels and pH in a planted aquarium is essential for creating a lush and flourishing underwater garden. By aiming for a slightly acidic pH of 6.5 and maintaining a CO2 concentration of around 30ppm, aquarists create optimal conditions for plant access to vital nutrients and discourage algae growth. Regular monitoring of alkalinity and CO2 levels, along with proper filtration and fertilization, contribute to a healthy and balanced ecosystem. By striking the right balance with CO2 injection, aquarists can ensure a stable and thriving aquarium environment, providing joy and beauty to their aquatic world.


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