15.6. Carbon Dioxide

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Author : David Bogert

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Unless one has some very easy aquarium plants (anubias, Amazon sword, Java fern, Java moss) a planted aquarium needs a source of carbon dioxide.

Carbon dioxide (CO₂) does help a great deal with any planted tank. The Aquapros YouTube channel (great channel!) ran a test of CO₂ injection. For 40 days they ran two aquariums side by side. The aquarium on the left had no CO₂ injection while the aquarium on the right had CO₂ injection. The results are rather obvious:

Side by Side Test of CO₂ systems in two Aquariums (from Aquapros YouTube Channel)

What is the Optimum CO₂ level

High tech planted aquarium enthusiasts use 30 ppm as the “gold standard” for carbon dioxide concentration in a high tech planted aquarium. If you have a low-energy planted aquarium, such as low lighting and nutrient levels, then 30 ppm CO₂ is not necessary. The plants will be happy with 10 to 20 ppm and this presents less risk to fish.

Also note that if one wants, you do not need to do carbon dioxide injection in a planted aquarium. You simply have to NOT do a lot of aeration or surface water movement. Aeration and surface water movement removes carbon dioxide from the water before the plants can use it. So it will slowly kill most plants.

Astronotus ocellatus Oscar Veiltail
Astronotus ocellatus – Oscar Veiltail

The low tech planted aquarium (low level CO₂), low tech pelleted aquarium (wood pellets under gravel), or Walstad aquarium (woody soil under coarse sand) are good options for adding some CO₂ to planted aquariums without any significant aeration. These will take the CO₂ levels from 3 ppm to 10 to 15 ppm, which is very good levels for most green plants.

These are a very good option for many people who want planted aquariums. Just don’t put any large fish  (over two inches) or a lot of fish in the tank with the plants, as without aeration the fish will die. Also do not use high intensity lighting.

Note I have never had success with any planted aquarium where there is SOME source of carbon dioxide to bring the CO₂ levels up to at least 10 ppm. But that just might be my ineptitude.

Pseudomugil gertrudae Aru
Pseudomugil gertrudae Aru

The Design of Carbon Dioxide Injection

There are many ways to add carbon dioxide to a planted aquarium:

  • High pressure injection from CO₂ tank
  • High pressure injection from a citric acid tank,
  • Low pressure system from a yeast set up
  • The Low Tech system
  • Soiled tank (Walstad)(soil adds CO₂!)
  • Wood pellet tank
  • Fish food

High pressure injection systems inject carbon dioxide from a pressurized tank into mixing device in the aquarium. There are several types of systems. The system used most often is run off of carbon dioxide cylinders like 5 lbs CO₂ aluminum tanks that can be obtained over the internet from welding supply houses and sometimes in beer dispensing supply houses. The containers are shipped empty and have to be filled at local welding supply houses.

Hemichromis bimaculatus Red Jewel Cichlid
Hemichromis bimaculatus – Red Jewel Cichlid

This “High Pressure” CO₂ injection system is the system used by most of the “pros” for high tech planted tanks. These systems add carbon dioxide gas (CO₂) to raise the carbon dioxide level in the planted aquarium from 2-3 ppm up to 20 or even 30 ppm.

Another high pressure system (Fluval) uses small cylinders of CO₂ gas. A closely related system uses a paintball CO₂ system. Another type of system uses citric acid and baking soda to create the CO₂. These are typically high pressure systems with pressure cylinders and pressure regulators. But there are also lower pressure DIY baking soda citric acid systems.

Copadichromis borleyi OB
Copadichromis borleyi OB

The only two systems I do not recommend are high pressure injection from a citric acid tank and low pressure system from a yeast set up. They are both just a constant hassle and not worth the effort.

All of these systems can be found in a raft of very good YouTube videos. So we won’t go into details on them. But note that the system we like is the Low Tech System. Details about it can be found at this link:

15.6.1 Low Tech CO₂ Aquarium System

Pseudotropheus polit Lion's Cove
Pseudotropheus polit – Lion’s Cove

Science of Carbon Dioxide and Plants in the Aquarium

This is largely based on an excellent article in Practical Fishkeeping:

Carbon dioxide (CO₂) injection in planted aquaria has become very popular and ever more manufacturers are supplying user-friendly kits. Carbon is the most important element for plant growth and gas injection is the most effective way to provide this nutrient. However, it is not a simple case of adding as much as possible. This gas is highly toxic when overdosed and is probably the biggest cause of livestock fatalities in high tech planted aquariums. It is therefore vital to test or monitor gas levels in the long-term.

A CO₂ content of 30 parts per million (ppm) or milligrams per liter (mg/l) is safe for most fish. However, the toxicity threshold can vary considerably, depending on dissolved oxygen (O₂) levels. In aquariums with relatively low levels of O₂, the fish will be more sensitive to the gas, but in aquariums with high levels of O₂ the safe threshold limit of CO₂ increases.

Practical Fishkeeping, “CO₂: Striking the Balance”, June 13, 2016

Because of something called the Bohr/Root effect high levels of carbon dioxide in the water reduce the oxygen carrying ability of the fish’s blood. Above very roughly 30 ppm carbon dioxide this CAN become fatal in some sensitive fish. This is the reason for the 30 ppm level cut-off. Note I like to use a 15 to 20 ppm cut off and I would never combine very expensive large fish like discus with high pressure CO₂ injection. High pressure CO₂ injection is notoriously unreliable and one can expect to lose a tankful of fish every once in a while.

Protomelas spilonotus Tanzania
Protomelas spilonotus Tanzania

Further Information on CO₂ Systems

More information on setting up a high tech CO₂ injection system can be found at this link:

15.6.3. High Tech CO₂ System

There is a low cost, low tech system which adds considerable CO₂ to an aquarium. The details of this system can be found at this link:

15.6.1 Low Tech CO₂ Aquarium System

Another type of aquarium uses a wood rich soil to provide a modicum of CO₂ to an aquarium:

15.8. Walstad Aquarium

Yet another type of planted aquarium uses wood pellets to provide extra CO₂:

15.10. Hybrid Planted Aquariums

Another system to add CO₂ to an aquarium is one where one uses food as the CO₂ source. This interesting method is examined in this link:

15.6.4. CO₂ from Food

Heros severus Red Spotted Severum
Heros severus – Red Spotted Severum

Measurement of the CO₂ level in an aquarium is covered in this link:

15.6.6. Measuring CO₂

The marketing “snake oil” claim of “liquid carbon dioxide” is examined in this article

15.6.5 Liquid CO₂

The intricate relationship between Plants, KH, pH and CO₂ is covered in this link:

15.6.2. KH, pH, CO₂ Relationships in a Planted Aquarium

And some additional data on the very complex Bermuda Triangle of pH, KH and CO₂ can be found in this link:

4.4.3. Carbon Dioxide and pH

Maylandia estherae Red Zebra
Maylandia estherae – Red Zebra