What CO2 can do for you (and your plants!)


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Fancy giving your plants a real energy boost? George Farmer takes a detailed look at what CO2 injection can do for the growing potential of your planted aquarium.

I started my first planted aquarium in 2003 and it was a dismal failure. Most of the plants were non-aquatic and I was given some pretty poor advice. Fast forward one year and I learned about CO2 injection!

I made my own yeast-based CO2 system and never looked back. Growing plants became easy and I was throwing away unwanted handfuls every week. As I became more experienced I invested in a high-end pressurised system that’s still running today.

CO2 injection is considered essential for many species and it will improve growth in the easiest plants too.

Combined with appropriate lighting and other nutrients, if you get your gas levels and distribution correct you can easily grow whatever plants you want.

Why injection?

Carbon dioxide is essential for the growth of plants. They use it as their main source of carbon and if the level is deliberately increased via injection then growth will increase accordingly.  

Converting the gas and light into plant growth is known as photosynthesis and, with appropriate lighting, injection and other nutrients, this process, known as pearling, can be visible as oxygen bubbles forming on the plant leaves.

Even a small increase in CO2 will improve plant growth, but the level must remain stable while the plants are under illumination, otherwise algae will proliferate.  

In aquaria with high lighting levels this gas is essential, plants being otherwise unable to cope with the rate of photosynthesis demanded. This, in turn, leads to poorly plants and algae issues.  

The same principle can be applied to other nutrients. With good light and CO2, plenty of other nutrients are necessary to feed the plants effectively, promote healthy growth and prevent algae.

CO2 is toxic to livestock so never add too much.

In the planted tank hobby overdosing is the biggest killer of fish and shrimp, so monitor levels effectively. Pressurised cylinders contain the gas stored at almost 1,000psi, so keep them well out of reach of inquisitive children and any pets.

What type of CO2 system is for you?

There are three commonly available injection methods:

Yeast-based: This relies on a mix of yeast, sugar and water to create gas as a by-product. This happens in a sealed container, like a soft drinks bottle, and the CO2 is delivered to the aquarium via an air line or tube.

Only by altering the ratios of your ingredients can you change the gas output. Ambient temperature also plays a part — the warmer the mixture, the more is produced, but for less time.

Pros: Inexpensive to set up and run. Suitable for smaller aquariums.

Cons: Uncontrollable and unstable CO2 levels. Unsuitable for larger aquariums.

Aerosol: Squeeze the trigger of this small disposable pressurised container to deliver the gas. A CO2 diffuser sits inside the aquarium and fills with gas. Release the trigger once the diffuser is full, then for the next few hours the gas diffuses into the water. Once the diffuser reservoir is empty re-use the aerosol.

Pros: Inexpensive to buy. Easy to use. Suitable for nano aquariums. Needs monitoring. Can’t be left to run unattended for long periods.

Cons: Unsuitable for larger aquariums. No adjustment possible.

Pressurised: A basic system consists of a pressurised cylinder, regulator and diffuser. Solenoids can be fitted to have the CO2 switched off at night and a pH controller can maintain a stable level.  

A needle valve and bubble counter can determine the rate of the CO2 being added and a non-return valve prevents any water damaging the regulator.

Regulators can vary. Higher-end models have two dials; a contents gauge and a working pressure gauge that can be adjusted. A full cylinder will contain the gas at around 60 bar (870 psi) and the regulator reduces this pressure to a working one bar (14.5 psi).  

Some diffusers need a higher working pressure due to their tight, porous ceramic structure. The gas is then delivered to the aquarium via some form of diffuser or reactor.

Pros: Controllable and stable. Customisable for any aquarium size. Can be fully automated. Good value if using refillable cylinders.    

Cons: Initially more expensive. Can be easy to overdose. Potentially dangerous high pressure levels.

Diffusers and reactors

The diffuser or reactor is final stage in any CO2 kit, responsible for getting the gas into the water. There are many designs suitable for all sizes and specifications of tank are these types are the most regularly used:


This is perhaps the most basic diffuser. Gas bubbles enter the bottom of a ladder-type device, rise through the rungs and dissolve into the water. The ladder also doubles as a bubble counter and at a glance it’s obvious how much CO2 has been injected. Expect to pay around £10 or more.

Ceramic disc

Gas builds up behind a ceramic disc which has tiny pores that allow a small path of gas to flow through. When underwater this produces CO2 micro-bubbles or mist, and the smaller bubbles the better as these dissolve more easily.

However, they are fragile so be careful when handling and disc quality can vary from model to model. Some can emit micro-bubbles from only a small proportion of surface area and bubbles can be large. Others will emit a consistent quantity of tiny ones throughout the surface.

There’s no guarantee which you’ll get, so buy several of the cheapest models to ensure that at least one will be good for you.

Ceramic discs are unsuitable for most yeast-based and aerosol systems as will they require a higher pressure.

Clean them by soaking in bleach or a limescale remover weekly to ensure the pores don’t get blocked, resulting in poor or large micro-bubble production.

Prices range from £3 to more than £100.

Inline diffuser

These are becoming the diffuser of choice for many in the hobby.

The principle is the same as the ceramic disc, but the device is installed inline with the filter outlet.  

An external canister filter is required. Cut the outlet hose near the outlet end, spray bar or lily pipe, and position the diffuser by attaching it inline.  

The gas hose is attached to the diffuser with the micro-bubbles being immediately picked up by the flow and then blasted around the aquarium.

There’s little cleaning required with this method because the device is on the filter outlet and little or no light reaches the ceramics, so preventing any algae build-up.

Expect to pay in the region of £15.


These are also fitted inline to an external filter, being relatively large and usually positioned inside the aquarium cabinet. They work on the same principle as the inline diffuser, but instead of micro-bubble production the gas is completely dissolved in the reactor. They are usually filled with bioballs or similar to help dissolve the gas.

There are no visible bubbles produced, but many models can restrict flow.

Expect to pay around £50.

CO2 and your fish

Up to 30ppm CO2 is considered safe for most fish and shrimp, but some species are more susceptible to intoxication than others. Fish from oxygen-rich environments are usually the most sensitive.  

Ensure oxygen levels are good, as well as gas levels. I do this by ensuring there’s always a good amount of surface movement on the water. You will use more gas, but the trade-off is worth it for the sake of your fish.  

It’s also worth investing in a solenoid if you have a pressurised system. Have it switch on your CO2 one hour prior to lighting and off one hour before the end of the photoperiod. This way the gas should be at a level to promote growth as soon as the lights are on and suitably low when the lights are off.

At night the plants produce CO2 and use oxygen, so observe your fish late at night and first thing in the morning, especially when first setting up a system.

When adding new fish to an injected aquarium consider turning off your lights and gas 24 hours prior to addition. This will ensure that the fish won’t be shocked by high gas levels.

Large water changes are the best way to quickly reduce CO2 levels in the event of emergencies.

Measuring CO2

Test the aquarium water for pH and KH and cross-refer the results to a table for a CO2 level.  

However, using the table assumes that the only acid in your water is carbonic acid that’s produced from the gas, when many other factors affect aquarium pH.

For this reason another testing method has become popular using a CO2 indicator or drop checker.

This device sits inside the aquarium, being filled with a solution of water that’s exactly KH 4 (4 KH water) and a pH reagent called bromothylene blue (also found in low-range pH test kits).  

The gas reacts with the solution and changes colour according to how much is present.

The aim is to get a nice lime green colour which equates to 30ppm CO2, which is regarded as safe in most situations.

If you have a low-range pH test kit then look at the colour that pH 6.6 gives. That’s the green you’re after in your drop checker.

Expect to pay around £5 for a drop checker, £5 for a low-range pH test kit (bromo blue) and £5 for some 4dKH water.

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