How Often Should I Put CO2 in my aquarium?


Carbon Dioxide is the most essential element for an aquatic plant’s survival.

All plants require Carbon Dioxide. Some plants may require it more than others but they all require it nonetheless. This is the primary reason why we use Carbon Dioxide supplementation in our planted aquariums. It is basically life support. A lot of doubt and confusion seems to revolve around this subject though given that Carbon Dioxide in large doses can kill fish faster than ammonia does. To shed some light on this subject this article will try to dissect CO2 in aquarium further.  


The difference between a low tech and a high tech tank set up is just the CO2 supplementation. High tech planted tanks have additional CO2 injected in the water column due to the increase of plant growth. Low tech setups would rely on the CO2 already present in the water or the CO2 that is dissolved in gas exchange, this will also result into slower aquatic plants growth in the planted aquarium.

           So how should I supply CO2 in the planted aquarium or more importantly how often should I supply CO2 in the planted aquarium? CO2 is only used by aquatic plant/s during photosynthesis (the process wherein the aquatic plants manufacture their own food). During this process they break down CO2 and convert the carbon into sugar primarily used for the plant’s energy. The excess Oxygen they will release into the atmosphere. During this process CO2 should be made readily available. CO2 levels in the water should be sufficient all throughout the course of photosynthesis.  

Normally a planted aquarium should receive around 8 to 12 hours of photoperiod (lights are turned on).

This would be the time where plant/s will photosynthesize. This is also the time they will be needing CO2. The fact is, plant/s do not breathe in CO2 when there is no photosynthesis going on. They breathe Oxygen at night. It is during the morning or when the lights are turned on that the carbon doser should be turned on as well. If using standard CO2 tanks, it would be best to connect it to a timer set to turn on at about an hour before the lights will be turned on. It should be set to turn off an hour before actual lights are turned off as well. The logic behind this is that the CO2 supplement should be at sufficient levels already when the lights are turned on. So the planted tank should have CO2 supplementation an hour early to ensure that the dissolved CO2 will climb to sufficient levels. It should be turned off an hour before lights are scheduled to be turned off since that CO2 supply should be sufficient enough until the lights are actually turned off.  

aquarium by Chriss Oddy

           So, how to add CO2 to an aquarium? This is a question with no definitive answer since each plant varies in their CO2 requirements. Planted tanks that has fast growing, high lighting loving plants would normally need more of the gas compared to the shade loving, slow growing plants. These are considered as high tech plants. So, a tank with high lighting would need more of it and a tank with medium to low light aquariums would need less. To complicate things, there are those plants that can tolerate low levels of CO2 even in high light. Such is the case with true aquatic plants. With these plants grow in depending on what you'll be able to provide for them. Generally, a 60 cm highly lit tank would need around 3-4 bubbles per second of CO2 and a low-lit tank should need just around one bubble per second or sometimes even less. Do not worry! There are ways to determine if the aquarium plants are provided with sufficient CO2 or if CO2 is too much. CO2 drop checker were designed specifically for this.  Those out on the market have a solution that will change colors depending on the level of dissolved CO2 in the tank. These readings are 1 hour delayed though so do not be too confident. It is always good to observe the fish respiration for any labored breathing or change in behavior.  

A tank with sufficient CO2 should have pearlings.

The term pearlings are those bubbles on the tips of aquatic plants. They are pure oxygen being released as a by product of photosynthesis. Pearling should occur 3 to 4 hours after lights are turned on and not an hour before lights will be turned off. Pearling should continue until the lights are turned off. If the pearlings appear much later, this would be a good indication that there is insufficient CO2 in the planted tank. If the pearlings appear almost as soon as the lights are turned on, that would mean there is too much CO2 in the tank. Pearlings will not appear or would be very difficult to spot if the water flow is too strong though so make sure that the flow is just right as well.

pearling carpeting plants

The amount of CO2 to be supplied is dependent on a lot of factors. Temperature, Water Hardness and the actual diffusion all contribute to attaining the right CO2 levels in the planted tank. Larger aquariums would require more CO2 injection.


Colder waters have the ability to dissolve and hold CO2 far better than warmer waters. The tank should be kept at 25-28 degrees Celsius. This would be warm enough for plants to thrive and achieve plant growth in and cold enough for CO2 to dissolve properly.

Soft water can dissolve and hold CO2 and other nutrients far more than hard water.

Water has a certain threshold when it comes to how much gas or solids it can dissolve and hold. This ability can be measured in the Total Dissolved Solids (TDS) readings. There are digital meters meant for this. Keep TDS at 80-150 so it can dissolve and hold CO2 better.

How CO2 is being diffused actually plays a huge role on how much CO2 will be supplied to the tank. The process would be to have the largest surface area of the gas exposed to water for as long as possible so water can dissolve it. This can be done by making the gas bubbles into smaller bubbles. The smaller the bubbles are, the better the dissolution rate would be. Some gadgets like the atomizer can create microbubbles so long as there is enough pressure. Apart from inline diffusers which also require pressure, this gadget is a very good way to get the CO2 dissolved in water. The inline diffuser holds the CO2 gas against rushing water and it does not allow it to escape. The gas will have no other way out but to be dissolved in the water. This is the best way to diffuse CO2 in aquarium water. Dissolution rate is almost perfect with this gadget. The only problem is, they can only be used with Canister filters.

Water flow is another factor that needs to be present in a planted tank. A CO2 supplemented tank will have uneven levels of CO2 without proper flow. This would mean that the plants far from the actual CO2 diffuser will not be able to get sufficient CO2 while on photosynthesis. It is really important to have those CO2 rich waters circulate all throughout the tank.

It would really not matter if the tank is being supplied with tens of bubbles per second of the CO2 gas if these factors are not correct in the tank. The gas will just simply float up and escape into the atmosphere.   

There are 3 common types of CO2 injection/supplementation. Biological, Chemical and of course, the commercial CO2 tank which is a great CO2 booster. The first two are D.I.Y. systems. It relies on the system being able to generate its own CO2. The third is pressurized CO2 in a container (tank).  All three have different uses and different ways. The biological generators will use yeast and sugar mixture. The yeast eats the sugar and produces CO2 and alcohol as by-products. CO2 in these systems is supplied 24/7. There is no turning this off. This will only stop when the yeast finally succumbs to the levels of alcohol. In this instance, it is best to keep CO2 supplementation at a much lower rate than what is regularly supplied in a tank. If a 60 cm tank receives 3-4 bubbles per second using the standard CO2 tank, keep the biological at 2 bubbles per second since it is supplied 24/7 anyway and the CO2 will rise to sufficient levels once lights are turned off.

The last 2 systems are supplied in the same way. Chemical systems can be turned off. Standard CO2 tanks can be turned off as well so these two should be turned on only when lights are turned on and turned off when lights are turned off as well.


Always keep in mind that CO2 is needed by plants when photosynthesis occurs. There should always be a source of CO2 in the tank. CO2 does not have to be supplemented as in the case with low-tech setups. CO2 supply, light intensity and nutrients should go hand in hand. When lighting intensity goes up, CO2 supply and nutrients should go up as well. There is absolutely no sense in supplying a low-lit tank with high amounts of CO2, same like putting activated carbon in planted tank arranged in Blackwater-style. Not only is it impractical but too much CO2 in aquarium will kill the fish as well.

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