Aquarium CO2 - Updated

by Tim Burton

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What is CO2?  Carbon dioxide gas (CO2) is without a doubt the most important factor for establishing a healthy planted aquarium with healthy plants and lighting close behind.  It is a waste gas that almost all organisms produce, and it is required by all photosynthetic organisms (plants, algae).  Without CO2, algae and plants cannot complete photosynthesis, which is the conversion of inorganic carbon (CO2) to organic carbon (glucose).  Photosynthesis is the process which allows plants to capture energy to grow.  Hobbyists must supply CO2 for plants to grow. In some cases, low light aquariums do not need supplemental CO2, as the amount fish supply is enough.  A general rule of thumb to determine if your tank requires CO2 is approximately 1-1.5 watts of normal output fluorescent light per US gallon of water.  If you have more light, or are using power compacts (PC) or high output T5’s, you will also certainly require some additional CO2.  To read more about non-CO2 aquariums, please read our article on Non-CO2 tanks.


As you read through these articles, you will notice that for someone living in a metric country (Canada), I tend to use inches and gallons as well as centimetres and litres.  For some reason, imperial units are more intuitive to me; I will try to use metric with imperial, but I’m certain I will not catch every instance.  Why use gallons for an aquarium?  Gallon just seems more natural to me.  On with CO2 for aquarium use…


To achieve the beautiful and gorgeous aquarium we all want, I highly recommend using CO2.  This article will explain how to use CO2 safely and effectively.  After all, bottled CO2 is under about 600-800 psi, and that is a lot of stored energy to accidentally discharge in your house.  Accidents are best avoided.


CO2 also gives us a tool with which to control nuisance algae.  The exact mechanisms are not perhaps not fully understood, but aquariums that maintain stable and slightly excessive concentrations of CO2 will not have algae issues (assuming of course that ammonia/ammonium levels are very, very low-more on this later).  My interpretation of this fact, gleaned from trial and error and learning the science behind Tom Barr’s estimative index approach, is that plants are very complex organisms that continually adapt to their environment.  When plants are forced to adapt, they must respond in various ways, and when stressed by environmental changes, they will release various chemicals, including ammonium (NH4+).  Ammonium is a powerful signal molecule to nuisance algae to begin a growth cycle (in natural systems, ammonium is a by-product of decay, which often indicates to algae that other nutrients will be available in large quantities, and so they respond by blooming).  Thus, plants that are not growing very well will actually stimulate algae to grow.  Therefore, the focus of the hobbyist should be to grow the plants well and algae will not be a problem.  This is a very incomplete understanding, because a lack of CO2 will cause one type of algae to grow (BBA), and directly adding ammonium to an aquarium will cause green water algae.  However, the main point to take from this is that adding CO2 in correct quantities is a must!


So how much CO2 should be added to an aquarium?  In a low light aquarium (approximately 1.5 watts per gallon or less), fish respiration is most likely sufficient for plant growth.  However, as lighting increases, so does the plants' requirement for CO2, and fish respiration is not sufficient.  In this situation, we must add CO2.  Aquarium plants cannot thrive without it!  There are numerous methods of supplying and injecting CO2, which we will discuss in an article specifically regarding current CO2 Equipment.  Generally speaking, we recommend 30 ppm CO2 (ppm = parts per million = mg/L).  This concentration will give plants enough CO2 to grow without limitation and stress, and will not harm any known tank inhabitants. Measurement of CO2 can be problematic however.  The best method is to observe the plants and notice if they are healthy.  The internet, and this website, have lots of pictures of healthy plants.  Healthy plants are vibrant, have no holes in their leaves, and they will ‘pearl’-produce oxygen bubbles- if they have adequate to high light.


However, since most beginners do not have a feel for such things, there are two methods that can work.  The first method is to use what hobbyists call a drop checker (DC).  There are several manufacturers that produce these –ADA, Red Sea -This method uses a pH indicator, usually bromothimol blue, to give a colourometric reading to indicate if CO2 is low, correct, or high.  A small clear bulb is filled with a reference solution with a known amount of carbonate (KH 4) and bromothimol blue.  This bulb is placed into the aquarium.  CO2 from the aquarium will then diffuse through the air gap and into the bulb and react with the dissociated carbonate.  This will lower the pH in the bulb, and the bromothimol blue will change colour, which the hobbyist can see.  Too much CO2 will lower the pH, and the colour will appear yellow; the correct amount appears green, and too little appears blue.  Red Sea and ADA both make their versions of drop checkers. The DC method is good because it is not affected by the water conditions of the aquarium.  Its main drawback is its slow response time (usually around 1-2 hours).  This means that even if you see a green colour, indicating good CO2, which may have been from 2 hours ago.  Another issue we have found is that CO2 can form a bubble underneath the air gap, giving false high readings.


The second method is using a pH controller.  This is a more high tech approach, and in principle works well.  A pH probe measure the pH of the aquarium, and the controller opens and closes a solenoid valve to release CO2 into the aquarium when the pH rises too high.  The hobbyist selects a set point based on the KH of the aquarium.  However, because this method is dependent on pH of the aquarium, any added chemicals that adjust pH will render the settings inaccurate.  For example, adding phosphates will lower the pH of the aquarium, giving a falsely high CO2 concentration measurement. So, until accurate and fast CO2 measuring devices are available for reasonable prices (several direct measurement CO2 probes are available, but retail for more than the average hobbyist is willing to spend), hobbyists can use either or both of these methods to get the CO2 into the correct ball park, but still must monitor their plants' growth.  Healthy, vibrant growth is a clear indication that CO2 levels are correct.  This observation method is definitely the least technical, but it is the most accurate, and sitting in front of your aquarium enjoying your plants grow does seem like a good method!


Since undoubtedly someone will ask me what my personal opinion is, I will write it here.  When I set up an aquarium for myself or a client, I use all three methods.  I put in a drop checker because it will tell me if the CO2 is in the right ball park.  The CO2 controller I use as well, but I never rely on the CO2/pH/KH chart, as this chart has told me in the past I was adding over 150 ppm CO2 to my tanks!  I add plants, turn on the CO2, and adjust the pH set point by monitoring the plants.  This process can take 2 weeks or more, but it is certainly the best method available to hobbyists.  Sometimes I will cheat by using a CO2 membrane probe; these probes are sold by point 4 systems, and are certainly not meant for the average hobbyist-think thousands of dollars each-but for someone like me who grows aquatic plants as a business, I have access to these toys.  This kind of CO2 equipment is not needed for the average aquarium CO2 system.


As a final comment on CO2, plants only utilize CO2 during the day.  Hobbyists using pressurized CO2 can conserve CO2 at night by using a solenoid valve on a timer.  Set the timer to come on about an hour (or more, if it’s a large tank) before the lights, and the CO2 will have reached the correct concentration by the time the lights turn on.  By turning CO2 off at night, the hobbyist will reduce the chance of gassing the fish when the plants stop absorbing CO2 when the lights go out.



Please feel free to link to this page. If you have an original aquarium-related website and would like a linkback, please email me at