The need for CO2

CO2 is arguably the most important element in the planted aquarium. It is required for respiration and growth by all aquatic plants, used in a process called photosynthesis. Plants require a constant supply of CO2 during the light hours, otherwise they can suffer. They combine the CO2 with water and light energy to produce oxygen and sugars to enable growth.

In the wild, plants get their CO2 in large quantities naturally from substrate (mud) and degrading plants. However, in an enclosed aquarium, CO2 is very limited. Water from your tap is depleted from CO2 and plant decay in an aquarium is minimal compared to the wild. This is why many aquarists have found out that supplementing CO2 really does help their plants grow much better and a lot stronger.

Deciding whether you need CO2 injection or not, or how much you need, depends on the amount of light you are supplying, and the selection of plants you wish to grow. However, for a more successful planted aquarium, we always recommend injecting CO2.


In low light aquariums, CO2 is not always necessary. Plants are less stimulated to grow under low light,  so extra injected CO2 is not required as there is normally enough CO2 supply coming from surface agitation, fish respiration and organic breakdown of dead plant matter. However, adding CO2 in a low light tank will still improve the quality of growth and health of your plants.

In aquariums with medium to high lighting, CO2 injection becomes vital. With more light available to the plants, the quicker they grow. This results in a higher demand for CO2 by the plants. Under medium to high lighting the aquarium becomes CO2 limited. The aquarist must now start adding CO2 to meet the plants demands. If the aquarium remains CO2 limited, your plants will suffer from growth deficiencies, and as a result you will experience algae formations.

The natural CO2 level in an aquarium is 0-0.5 mg/L of water.
Even the slightest addition of CO2 can make a big difference improving the growth and vigour of the plants.
If the aquarium contains only slow-growing Easy plants, the addition of CO2 gas into the water should reach a level of approx. 3 mg/L to ensure good growth conditions.
For Medium category plants the CO2 requirement is approximately 6-14 mg/L and for Hard plants approx.  15-25 mg/L.

Top tips:

• Turn your CO2 OFF 1 hours before the lights go out. There will be enough remaining CO2 in your aquarium for your plants during the last hour of your photo period. This will save you on CO2 consumption.

• To start with, set the solenoid to turn ON the CO2 1-3 hours before lights come ON (may require a longer period in larger tanks). This will ensure the CO2 levels in your water are brought up to optimum concentration for the plants when the photo period starts. The most important time for your plants when they photosynthesize is at the beginning of the photo period. Aim for a nice green colour on your drop checker at the start of the photo period.

• You may be required to do a little experimentation with your bubble count and CO2 on/off times. This is because every tank is different and will require differing rates of CO2 injection to achieve a green colour on your drop checker.

• Be careful not to inject to much CO2 to high levels as this could harm your fish. Start low and work your way up to a optimum level for your fish and plants.

• It is recommended to experiment with CO2 levels before introducing fish to the aquarium. This will avoid putting the fish at risk whilst you find the 'sweet spot' with your CO2 injection.