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CO2 In the planted Aquarium

Discussion in 'Tutorials' started by aaronnorth, 13 Jul 2008.

  1. aaronnorth

    aaronnorth Member

    Carbon Dioxide (CO2)
    Plants are made up of 50% carbon (dry weight). Submerged aquarium plants absorb carbon through carbon dioxide (CO2) dissolved in the water. All aquarium plants will benefit greatly from adding CO2 to the water. For those wanting to keep a heavily planted tank, full of lush growth with little or no algae then I consider CO2 addition to be almost essential.


    Under low light conditions (generally less than 2 Watts per Gallon) there is normally enough CO2 for the plant to grow successfully without CO2 addition. If the light levels are increased to over 2 WPG then the plants will grow well for a period until they consume the little CO2 (normal levels are around 6ppm) that is available in the tank. The plants growth is then limited by a lack of CO2. The plants will stop growing, resulting in an algae bloom because of the light and suspended nutrients. Algae’s biggest friend is a tank where there is lots of light and little plant growth. It therefore becomes necessary to increase CO2 levels within the aquarium. CO2 addition (injection) is therefore recommended with light levels of 2 WPG or over. The high lighting ‘drives’ the plants to uptake more CO2 and nutrients (see Lighting article).

    The cheapest and a popular way is to DIY. DIY generally uses an old plastic fizzy drinks bottle with tubing. The bottle is filled with a mixture of sugar, yeast, water and bi-carbonate of soda. The fermentation of the yeast produces CO2. The CO2 goes through the tubing that is normally attached to a diffuser or under the filter inlet within the tank. The CO2 must be diffused effectively in the water, it cannot just be allowed to escape from the tube directly into the water otherwise the CO2 bubble will just rise to the surface out of the water. Here is a guide on how to build a DIY CO2 system:


    http://www.aquatic-eden.com/2006/10/bui ... rator.html

    There are manufactured CO2 systems that work using the same fermentation process. These are slightly more expensive but have the advantage of being easier to set up and come complete with an effective diffuser. The Nutrafin Natural Plant System is a good example. The manufacturer’s supplied sachets are simply yeast and bi-carbonate of soda. It is worth noting that bi-carb is unnecessary with water that has a GH of 6 or above and KH 4 or above.

    I would recommend to use more than one unit (DIY or manufacturer system) to achieve a more stable CO2. This is achieved simply by swapping the mixtures alternately, one at the weekend and one midweek. Also I would chuck the mixture supplied with the Nutrafin system as the yeast is produced to last longer than normal unlike baker’s yeast. Another option is to use brewer’s yeast which can support a higher alcohol content so therefore lasting even longer.

    Pressurized Cylinder
    Another more expensive method is the pressurized cylinder. A CO2 filled metal refillable or plastic disposable cylinder is connected to a pressure regulator. This brings down the pressure to usable levels and can be adjusted to achieve the desired flow rate using a needle valve. The CO2 hose is can be connected to a solenoid which can be timed to turn off at night to save CO2 as plants don’t use CO2 at night. It also stops the ph dropping to low which could be fatal to your fish (more on this later). Then to a bubble counter (BC). The BC counts the bubbles produced per second (BPS) or per minute (BPM).A safety check valve is fitted to ensure no backflow of the aquarium water goes into the solenoid or Gas canister. Finally the CO2 flows to a diffuser or reactor. Initially these are expensive, these will provide a constant CO2 flow for many months (depending on tank size, planting density etc). A pH controller can be fitted, this will continue to inject CO2 until the pH drops to a preset level. A solenoid will then shut-off the CO2 supply until the pH has risen to the preset level. This provides a very stable pH. However, the downsides to pH controllers far outweigh the advantages of a stable pH (which isnt actually necassary in itself!). CO2 injection will drop the pH by approximatley 1 unit - fish experience pH swings much larger than this in the wild, one example is acid rain. Also, the CO2 being switched on and off constantly causes instability which may lead to algae problems. Finally, a pH controller may actually limit carbon to the plants. For example, you decide you dont want the pH to drop below pH6.5, and your tap water is pH7. Before you can inject 30ppm of CO2, the pH controller will cut it off due to the pH being to low!! you may only be able to inject 20ppm in that time period.

    Liquid Carbon
    Becoming increasingly popular is liquid carbon. It isnt actually carbon, but a complex carbon compund made up of glutaraldehyde (C5H8O2) as a base foundation, this goes though various chemical proccesses and it is fixed into carbon using the calvin cylce. Liveworts, Bladderworts & algae do not have this function, so instead it becomes toxic to them. most plants are fine with the reccomended dose, but then they rot if they are raised. Particular species include:
    riccia fluitans
    utricularia graminifolia
    egeri densa

    Liquid carbon is an excellent alternative to other CO2 methods, but it is worth mentioning it isnt as effective as CO2 gas alone. It can be dosed alongside other types of CO2 injection methods. It is very cost effective on small tanks.

    Yeast is Cheap to set up, but may cost as much or more in the long run, it is ideal for smaller tanks and easy to set up. There is also little chance of overdosing CO2. Some disadvantages of yeast based system is that there is unstable CO2 levels compared with pressurized which could cause algae, there is high maintenance (changing mixtures, cleaning etc.) and little control over CO2 output, not ideal for larger tanks as lots of bottles are needed.

    With pressurized, you get very stable CO2 (especially with a solenoid), it is easily controllable, it is low maintenance (once set-up it will last months depending on size of tank and plant mass), it is ideal for all sizes of aquarium, no matter what size, whether it be 1 or 1000gallons.

    It is expensive initially but cheaper over the long run and there is a possibility of overdosing CO2 but it is easily adjusted to bring it under control. Once the pressurized system is set up, the pressure may drop so it may need tweaking a little but it should be rectified within 48 hours.

    You can also do a DIY pressurized system, as long as you leave the safety pin in, dont press the handle, and don't knock it over then everything is ok.

    Fire Extinghuisher CO2

    Courtesy of Sam (themuleous) for writing this excellent Guide.

    Recommended levels of CO2 are 25 to 35ppm. It is important that these levels remain fairly constant, as fluctuating levels are known to cause algae. Higher levels are sometimes used to fight off algae; it is known that levels higher than 40ppm can block the enzyme production in the algae cells. However you must watch the behaviour of your fish high levels can cause problems.

    One of the consequences of injecting CO2 into water is that it produces carbonic acid. This acid will reduce pH (generally by 1 on the pH scale) so it is important that the water has sufficient buffering capacity (Carbonate Hardness or KH). Ideally the water should have a KH of at least 2 degrees or 40ppm.
    However, people have reported no problems even when using 0dKh water!!

    using pH/ KH relationship charts is a very innacurate way of measuring CO2 levels. Tank water contains other acidic substances, which can affect the reading. Even Nitrate and Phosphate can!
    As the CO2 gasses off from the aquarium into the drop checker, the ph lowers due to the carbonic acids formed by CO2 when it dissolves in water. When the drop checker turns from blue, to green, this indicates a pH of 6.6, and because we know the solution is 4dkh, this tells us we have approximately 30ppm of CO2 in our tank water.
    If you are using a 5dkh reference solution, then a pH 6.6 (green drop checker) will indicate 38ppm.

    Another way of measuring CO2 is to use a drop checker (DC), You fill the ball with 4DKH solution (make it yourself or get it from Aqua Essentials ) You then add a couple of drops of bromothymol blue or low range pH test to the 4DKH solution. This should then turn blue. Turn it the correct way up and place in the aquarium, the air bubble (forms automatically) will stop the solution mixing with the tank water.
    CO2 measurment usng a Drop Checker
    A benefit of CO2 is that the reduced pH levels can often be beneficial to a lot of tropical fish. The majority of freshwater tropical fish originate from water with soft and acidic water, examples include Tetras, Rasboras, Angelfish, Discus, Barbs, Corydorus, Loaches, Killifish, Gouramis, Bettas and most species of Catfish i.e. Plecs.

    Blue - low CO2 (less then 20ppm)
    Green - Ok (between 20ppm & 35ppm)
    yellow - too much (over 35ppm)

    During the day (when the tank is lit) the continuous injection of CO2 causes the pH to drop as carbonic acid is produced. we usually use a solenoid to cut the CO2 off at night, which means the pH would then rise as the CO2 gasses off, but if you dont, then the pH will remain fairly constant.
    Injecting CO2 usually causes a drop in pH by 1 unit.


    CO2 is quite unstable in water and it must remain in contact with the water for it to fully dissolve. There are devices that help with this and these are called diffusers or reactors. There are many types of diffusers available ranging from a simple bell-jar, where the CO2 bubbles simply collect and dissolve over time. A more popular method is a ladder-type device where each bubble travels up through a series of rungs, the bubble remains in contact with the water for longer and the water absorbs the CO2. This process is visible as the bubbles get smaller as the diffuse into the water. The Nutrafin kit comes with a ladder diffuser.

    In larger aquariums the common diffusers are not really efficient and another method is the reactor. They are not self-driven and rely on a source of flow for the CO2 to dissolve effectively. Reactors are normally associated with pressurized CO2.

    Common types of reactors are fitted in-line with the external filter output. The CO2 is produced by the pressurized cylinder and flows into the reactor. The reactor is filled with some form of media that allows the CO2 bubbles to dissolve fully where the CO2 enriched water is then pumped into the aquarium.

    A really simple method of CO2 reaction is to use the external filter; you can run the CO2 output directly into your external filter inlet. The bubble diffuses through the filter media and the CO2 enriched water flows out into the aquarium. The only disadvantage is that it can make a slight noise when inside the filter. It is still quiet though.


    If you have over 2 WPG of lighting with a stable CO2 level of 20-35ppm then plant growth will be fairly rapid. Especially if the plants are a fast growing species (particularly stem plants). The rapid growth results in the plants using up various and more nutrients. These nutrients need to be replenished by a fertiliser; otherwise the plants stop growing leading to algae. Therefore fertilisers need to be added regularly.
    EI using dry salts

    Injecting CO2 in an aquarium no matter how much light you have will help, even low CO2 is better than no CO2. Please ask for specifc information on your tank in the planted section.
  2. beeky

    beeky Member

    Nice work, cheers for that.

    I'll mention here that the disadvantage of pH/KH/CO2 tables are that other acids caused by bogwood and decomposition can effect the results. The tables referred to assume carbonic acid is the only acid present. That's why drop checkers are increasing popular as the use of 4dKH water is more accurate. They can also be mis-interpreted by people summising that if they change their pH or KH it can effect how much CO2 is in the water.
    kisanjong likes this.
  3. Themuleous

    Themuleous Member

    Nice one Aaron :)
  4. LondonDragon

    LondonDragon Administrator Staff Member

    Nice write up Aaron ;)
    Paulo Soares likes this.
  5. Wolfenrook

    Wolfenrook Member

    Not bad, but you have a pretty hefty bit of falsehood right at the start:-

    Seeing as how plants are about at least 80% water, it would be a bit difficult to have 50% carbon in there as well. ;)

  6. aaronnorth

    aaronnorth Member

    i'm sure it could squeeze in :lol:

    I'll change it :)
  7. Brenmuk

    Brenmuk Member

    In the Ecology of the Planted Aquarium it states that aquatic plants are about 40% carbon, that is 400, 000 mg C/kg plant dry weight. When looking at feeding plants it is helpful to consider their composition in terms of dry weight so that we can get a better idea of the nutrients they need.
    You might want to change it back to your original statement but qualify it as dry weight :)

  8. aaronnorth

    aaronnorth Member

    ok, i can feel a big dispute coming on :lol:
  9. Wolfenrook

    Wolfenrook Member

    Not so much a dispute, but dry weight is misleading as to most this just means not wet. More accurate would be to say 405 carbon when dehydrated, as even when 'dry' the cells of plants still include about 80% minimum (most plants it's closer to 90%, it's actually animals that are about 80% water) water in the construction of their cells. I really couldn't care less if a book choses to make scientifically inaccurate statements, this still doesn't make them true, maybe the author needs to go back to school and do his basic biology again? ;)

  10. Egmel

    Egmel Member

    I disagree, usually when you buy something 'dry weight' means the weight without fluids, be that water or otherwise. I'd be quite happy with that term and if you want to be totally sure just link it to the wiki page as I've done. There's no point not using the correct term because it might be misunderstood, better to use the correct term and educate the reader, or is that just a perpetual student's view ;)
  11. vauxhallmark

    vauxhallmark Member

    Wolfenrook, I agree with your argument completely, except for the words "fluids, be that", and "or otherwise". Dry weight (in this context) does mean without water, not without any other fluids. Mercury at room temperature is a fluid, but completely dry. The same goes for dry cleaning fluids (!), petrol or the liquid CO2 in your cylinder (and if you froze that, it would be dry ice).

    You're right, though, that the term dry weight is the correct one, and, as you say, I think most people would understand it in the article above to mean "with all the water removed", and not "after wiping water off the leaves"

    Go perpetual students!


  12. Egmel

    Egmel Member

    Difficult one, it depends on what you're buying, cars and bikes are quoted a dry weight so that the fuel and oil's don't count. In the context of almost anything else though you're absolutely correct, it's without water. ;)
  13. vauxhallmark

    vauxhallmark Member

    Mmm, yes ... that definition (vehicles) is already listed on the Wikipedia page quoted by Wolfenrook two posts back. I specifically said "in this context" the first time I referred to the term, and "in the article above" the second time. Nobody could think that "dry weight" in the original post could refer to plants with an empty petrol tank, and unlubricated by moter oil ... or could they? ;)
  14. Ed Seeley

    Ed Seeley Member

    Dry weight in a scientific context such as this article refers to is generally the dessicated weight of a sample. To obtain such a weight the sample is dried at a low heat until it's weight ceases to drop. This is then a sign that all the liquid water has been removed from the sample. It can then be sampled and tested to determine the other constituents of the matter such as protein, fat, and other materials.
  15. aaronnorth

    aaronnorth Member

    do you think i should put the amount of carbon a plant is made up of in dry weight o just leave it?
  16. plantbrain

    plantbrain Expert

    In standard analysis of nutrients/atoms, all percentages are as dry weights, otherwise, how could anyone compare due to variation in water status? If you have an answer that's easy to do, I'm all ears, you can make a wet to dry weight correlation, but it depends on th methods and the plant and specific for that study and hopefully no psuedoreplication occurs.

    Most dry weights from macrophtyes fall into 40-50% ranges.
    Next largest is N at perhaps 1-2%.
    See http://www.bestwebbuys.com/Mineral_Nutr ... c=b-search
    Arnold Bloom is a past professor of mine here at UC Davis and the co author above.

    Having done and am doing dry weights several times a year for various projects, you can also make a dry to wet weight correlation factor as well. We typically dry most at 70C for 48 hours for most aquatic plants unless there's a really large thick tuber or something. Then you weigh it, grind it and send it to the nutrient analysis phase for % N, P, Fe or Carbon etc.

    For non destructive sampling, you take O2 readings and use that to relate to rates of growth.
    Or CO2 uptake relative to the location of CO2 meter.

    and so forth............
    So if you use your own over and can set it to 70 C fairly well and keep it there for 48 hours etc, you can do this, or if you have access or can bribe someone at a plant science lab to dry it for you, have access to an accurate scale, any hobbyists can do the weights.

    You sample, you dry and a you weigh.
    Not high tech or anything.

    But you need to have a standardized starting weight, so the correlation between wet weight starting values and dry weight is important, but again, this is not hard to do. You take 20 sub samples(or 5, 10, 100 etc however good you want your stat's to be that is practical), and weight the before and after dry weights, then use that ratio to gauge the differences in the starting weights of the live plants.

    So say you want to measure 100 plants' response to CO2 enrichment for 8 weeks. You need 120 plants/stems all about the same relatively. You can weight and measure their length(you can make a dry weight to length ratio as well), and take that 20 subsmaple and dry them BEFORE you treat and grow them out. This gives you the starting reference point.
    So this way you can make a relative rate of growth, often called RGR to compare across treatments without having to kill and dry your plants along the way.

    Plants get much larger as they grow and often in exponential fashion, so adding 20ppm when the tank is first planted might be fine, however, 2 months later, when the biomass is now 800% more and the tank is fairly full, you might need to add 7 X the same amount of CO2.

    If you trim and garden routinely, then this is not an issue, it's when folks neglect CO2 and the tank in general, let things go too long, that we see issues.

    This is especially true for higher light systems and where we use CO2 gas as well.
    increases the rates of growth and speeds everything up, which might not be your goal in aquatic horticulture(or might be).

    Tom Barr
  17. brian68

    brian68 Newly Registered

    I have a 200L tank and am injecting pressurised CO2. One thing I'm not clear on, and this is pretty basic, is the approximate values for bubble rate (eg, in CEG's excellent article about using a drop checker he suggests 40 to 60b gallon 1 bubble per second, 20 to 40 gallon 1 bubble every 2 seconds, 10 to 20 gallon 1 bubble every 5 seconds).

    Do these rates assume CO2 is running 24/7?, i.e. if you are using a solenoid to shut it off overnight should the rate be increased, or would this be unnecessary? I realise that the CO2 is not needed overnight, in fact is probably harmful, but should there be some compensation during the day?

    Also, is there any truth in the idea that turning the CO2 up a little bit would help to drive off algae (eg by boosting the plants ability to compete?)

    Thanks in advance.
  18. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

    Hi Brian,
    It's very difficult to advise on absolute numbers of bubble rates because bubbles differ in size. If you are running CO2 24/7 then you are limited in the injection rate you can use. Shutting off the CO2 at night allows you to inject at higher rates although this does not mean that 24/7 is automatically inferior. It depends on the tank size, flow rates, lighting and so forth. Bubble rates therefore are a means to an end. So it's more accurate to say that if have livestock in the tank and if you use a solenoid then you can more safely increase the injection rate compared to a 24/7 scheme.

    The initial bubble rates in the dropchecker article were just standard conservative values we agreed on that would not immediately annihilate the fish just starting off. Increase the rates as the plants grow or if you see CO2 related algae. I'm always making adjustments to the injection rate due to these factors as well as because the regulator I use tends to slow the rate as the pressure in the cylinder falls. Keep an eye on the plants and they will tell you if they need more CO2. If you see indications of CO2 related algae (hair, staghorn, BBA) or if you see indications of Carbon starvation (melting, browning, disintegration, leggy growth) then increase the rate.

    Try not to think of plant growth in terms of a competition between higher plants and algae. This mindset will always lead you down the wrong path. CO2 does not "drive off" algae because algae are plants too and they love CO2 as well. Higher plants cannot compete with algae for nutrients just like elephants do not compete with mice for their food. The needs of the plants are so much higher while the needs of algae are so much lower.

    An increase in CO2 availability means an increase in Carbon of which the basic structure (or building blocks) of plants are made. More structure means more growth and more health. In some ways it would be better to think of algae in the same way we think of hyenas or vultures. Algae attack when plants are unhealthy, and algae recede when plants are healthy, so increased CO2 generates healthier plants which then eliminates the conditions in which algae tend to attack.

  19. aaronnorth

    aaronnorth Member

    i dont go with the BPS etc.

    For my 60l, it is recomended 1 bubble every 3 seconds, but i inject 3 BPS!

    Could CO2 level be so high that it kills off the plant? I have read that CO2 above 40pppm can block the enzymes in algae and kill it.
  20. ceg4048

    ceg4048 Expert/Global Moderator Staff Member

    Well I've never killed a plant using CO2 but I have killed them not using enough CO2. If it can be driven to levels toxic to plants you're more likely to kill your fauna way before that happens. Which enzymes are we talking about? Plants produce thousands of different kinds.

    Scientists found that terrestrial plants vary the number of stomatal openings on their leaves as a function of the mean CO2 concentration in a given growing season. The stoma are the entry paths for gas exchange. The higher the concentration the lower the number of stomata. Fossil records have shown variability in the CO2 concentration based on the stomatal population within the leaf fossil. From what I recall, recent atmospheric CO2 concentrations have been somewhere between 280 ppm (pre-industrial age) to about 370ppm (modern day).

    That's why I don't buy the 40ppm lethal dosage figure. No way, no how. Can you see why dropping plants in a tank and then feeding them a measly 20-30ppm of CO2 causes them to object violently? :wideyed:

    Last edited: 25 Feb 2014
    Paulo Soares and Dan walton like this.

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