Is CO2 fertilization necessary in water with low carbonate hardness?

Discussion in 'Plant Problems' started by Dirk B, Jan 7, 2008.

  1. Dirk B
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    Dirk B Aquascaper

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    On a number of instances on this forum, I have made the statement that under soft water conditions, CO2 fertilization is not necessary, and I would like to explain why I maintain this.

    In order to explain this, it is necessary to understand the concept of pH, the effect of the solution of carbon dioxide in water on pH, and the effect of carbonate hardness on pH and free CO2 dissolved in water. Then I need to show you a table on free CO2 levels in water of different carbonate hardness levels and pH to explain my point.

    1. (pH) So, what is the measure of pH that we all talk about all the time? Water, H2O, is chemically very stable, but a very small proportion will break down (ionize) into H+ and OH- ions. One molecule of water will break down into one H+ and OH- ion, but these can reassociate just as quickly back to form water again. We write this chemically as follows:

    H2O ↔ H+ + OH-

    The forward and backward arrow indicates breakdown on the one hand and formation of water again if the breakdown products reassociate. One liter of water at any given time will contain 10-7 moles of H+ and OH- ions. The amounts of H+ and OH- ions will be the same or in balance or in equilibrium as we call it. This water would also be called neutral.

    What is important is that if you upset the balance or equilibrium, it will try to compensate. If you add free acid, H+ ions, the amounts of H+ and OH- ions will no longer be the same. The added H+ ions will react with the OH- ions to form water, but because there are more H+ ions than OH- ions this imbalance will remain and the water will remain acid. In exactly the same way, if you add alkali or OH- ions, the amounts of H+ and OH- ions will no longer be the same. The added OH- ions will react with the H+ ions to form water, but because there are more OH- ions than H+ ions this imbalance will remain and the water will remain alkaline.

    The pH scale for measuring acidity or alkalinity is directly related to this in that pH is a log scale for measuring acidity:

    pH = - log (concentration of H+ ions) which is therefore = 7 under conditions when the H+ and OH- ions are present in equal amounts i.e. under neutral conditions.

    Under acid conditions when the H+ ions are in excess the pH will be below 7 and under alkaline conditions when OH- ions are in excess the pH will be above 7.

    2. The effect of the solution of carbon dioxide in water on pH.

    If carbon dioxide dissolves in water, it will react with water to form carbonic acid, but the carbonic acid will immediately break up (ionize) to form acid and bicarbonate. Chemically we write this as follows:

    CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

    Now the effect of this CO2 addition therefore is that the amount of acid in the water increases and this then causes the pH to drop.

    One can illustrate this effect if you measure the pH of freshly distilled water, which will be 7. If you leave this water to stand overnight, CO2 will dissolve in the water and because of the formation of the acid, the pH will drop to 4!

    What I am saying is that addition of CO2 through CO2 fertilization will cause the pH to drop!

    3. The effect of carbonate hardness on pH.

    If water flows over rock rich in calcium carbonate, or lime, as it is found extensively in Europe (the white cliffs of Dover are solid calcium carbonate), the calcium carbonate will dissolve or ionize in the water. Chemically we can write this as follows:

    Ca(HCO3)2 → Ca2+ + 2 HCO3-

    If HCO3- (bicarbonate) dissolves in water it can react with the very low levels of H+ ions in the water to form carbonic acid which in turn will form water and carbon dioxide, in other words it reverses the reaction that I have indicated above:

    H+ + HCO3- ↔ H2CO3 ↔ CO2 + H2O
    .

    In other words this upsets the balance of the water dissociation reaction and uses up acid, leading to an excess of OH- ions, which means the water becomes alkaline. It also means that CO2 is formed and released from the water.

    It also means the more calcium carbonate dissolves in the water, the more alkaline the water becomes and the less free CO2 will be dissolved in it. We could also say that the ability to carry free CO2 in this water is very much reduced.

    4. The table on free CO2 levels in water of different carbonate hardness levels and pH.

    In summary, I can say that if carbon dioxide dissolves in water it will acidy it, and if calcium carbonate dissolves in water, it makes it alkaline and reduces the amount of free CO2 in the water. This is illustrated by the following table illustrating the effect of carbonate hardness and pH on free CO2 levels:

    [​IMG]

    Now, if you look at this table you will firstly see figures drop from left to right, in other words as pH increases dissolved CO2 levels drop. You will secondly see that figures increase from top to bottom, in other words as the carbonate hardness increases, so the dissolved CO2 levels increase.

    Remember that I mentioned that water that is high in carbonate hardness has a high pH, and if you look at the table on the bottom right hand corner, for example, you will see that at a carbonate hardness of 10 (which is quite hard!), you will that at pH 8 the free CO2 levels will only be 4.4.

    In CO2 fertilization the objective is to increase the levels of free CO2 by direct injection into the water, in other forcing the CO2 levels to the natural maximum amounts of CO2 that the water can carry. The effect of this large amount of CO2 is that it then reacts with the water and acidifies it, causing the pH to drop and this must therefore be regulated. If you do not regulate it, the pH will drop to levels that are dangerous to the fish. At the same time the plants will be absorbing CO2 from the water as they perform photosynthesis. However, at night or when the lights are switched off, no photosynthesis can take place and no CO2 will be used by the plants and the result will be that the pH will drop even more. Regulation of CO2 injection is performed by a regulator that measures pH, and the pH that one aims for is in the neutral range from about pH 6.8 to 7.2. This is not detrimental for the fish and enough CO2 enters the water for the plants to absorb.

    If you look at the Table and look at the carbonate hardness column, you will see that with a carbonate hardness of less than 4.0 and a neutral pH, the actual amounts of CO2 that water can absorb is relatively low. For this reason, the books that I have read on the topic recommend that the carbonate hardness of the water be increased to at least 3.5, which also means that the pH is more stable and can be regulated more easily. So hardening the water if you do have soft water coming from the tap, as is the case in the Western Cape is what the books recommend.

    Finally, the point that I want to make, is if you look at the this table and you look at a low hardness and a lower pH, let us say a carbonate hardness of 2 and a pH of 6.4, the amount of dissolved CO2 is 24 Mg/litre, which is exactly the same CO2 level present at a carbonate hardness of 8 with CO2 fertilization in which the pH is regulated to 7. Why then harden up the water and use CO2 fertilization in the first place? If your water is soft, I do not see the need for CO2 fertlization. One must remember that aquarists started using CO2 fertilization in Europe and the USA in areas which had hard water with high pH and in which the levels of free CO2 were too low. In my opinion there is no need to use CO2 fertilization if you have softer water?

    I would appreciate the opinion of others who have studied this subject in depth as I have over the past 20 odd years.

    Kind regards,

    Dirk
     
    Last edited by a moderator: Apr 11, 2014
  2. dart
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    dart Green fingers

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    Very interesting, thanks for that Dirk!

    I?ve certainly not studied this for 20 years, but I do have some questions ;)


    - Interpreting the table: As I understood things this table represents the ideal case. I.e.it is only true if there are no other factors influencing the pH. (Which is the reason that we use drop-checkers: so that we can test the CO2 dissolved in distilled water). This means that we can?t simply look at the pH and hardness and assume that it is dissolved CO2 that is forcing the pH down.

    But If I?ve understand correctly, you are saying that the table *does* indicate how much CO2 water with a given hardness can absorb. So if you have soft water and a low pH your water will absorb CO2 from the atmosphere through osmosis and you will thus not need to add CO2...

    My tap water (Johannesburg North) has a pH of 7.8 and a hardness of 4, which means that it only has 1.87ppm CO2. So are you saying that if I change the pH to 6.6, by adding acid, and assume that the hardness remains more or less constant, then my water would be able to absorb more CO2. And the reason that this will happen is due to the H+ + HCO3- ↔ H2CO3 ↔ CO2 + H2O reaction. Is that correct?


    - In one of your previous posts you said "pH changes force the kidneys to work very hard to maintain osmoregulation and cause problems in maintaining blood homeostasis (maintaining constant blood pH)."

    Many of the CO2 supporters (Tom Barr etc) tend to say is that pH fluctuations due to changes in hardness are indeed bad/fatal but that changes in the pH due to dissolved CO2 are not problematic precisely because there is no change in hardness/dissolved minerals.

    I assume that you are saying that pH fluctuations regardless of the cause are indeed bad for the fish. Is that correct?


    - Does anyone have any experience with/comments on Flourish Excel or Easylife Easy Carbo? Excel seems to be fairly popular overseas and I?ve seen some very positive reviews.


    Thanks again for taking the time to write the great post!
     
  3. Alwyn
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    Alwyn Algae harvester

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    This thread could have saved me allot of money before buying my pressurized c02 setup  :-[

    Thank you so much for this awesome thread!! I really only now understand the reason for a good quality test kit. In the beginning I used to love testing my water parameters (did not always know what I was looking for though  :-[ ), but as time progressed I go “lazy” and used the “if my fish and plants are happy, I am happy” method, but now I realize the potential of better life for my fish and plants with a bit of stability in my setup.

    My shopping list now include a (good) pH and KH test kit, thanks Dirk  :thumb:
     
  4. Dirk B
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    Dirk B Aquascaper

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    Hi Andre,

    Thanks for your comments, your deductions are correct, but I will explain a little more to each question:


    Yes, you are quite correct, if I understand you correctly, there can be other factors that can push the pH down. One of the major factors that push pH down on an ongoing basis is the formation of nitrate from the ammonia produced by the fish. This process releases acid in the water. If you simply continue feeding your fish in a tank and don't do any water changes your pH will go down steadily. In soft water this goes down faster of course than in harder water (greater carbonate hardness).

    Correct, but importantly if you aerate the water the CO2 will be flushed out of the water. So, you should not aerate the water or agitate it with a spray bar or a jetted outlet as this will remove CO2. Also, the amount of CO2 that does dissolve is dependent on the surface area of the tank. Fish will of course also produce CO2, and the plants will produce O2 which is good, but remember that at night no O2 is formed, so you have to be careful not to have oxygen shortages at night. But you are correct, soft water with a low pH will absorb CO2 through osmosis and you should not need CO2.

    Perfectly correct!

    Again, correct. I do not agree that pH fluctuations due to dissolved CO2 only are not problematic. My water here out of the tap is soft, right. Some of my systems (discus and angels) run at a low pH due to acidication as a result of high ammonia production (see above), whilst other systems I keep at a higher pH through water changes (my tap water comes out of the tap at pH7.5). So the hardness of all of these systems is the same. If I move fishes from pH 6.5 tanks to pH 4 tanks or vice versa withour careful and slow adaptation over a few hours, they go into pH shock and die. It must be remembered that pH changes have dramatic effects on blood pH in a fish as the acidity is absorbed directly through the gills. The buffering capacity of the blood therefore has to work very hard immediately after moving fish from water with one pH to another, and the kidneys have to work hard as well to adapt the osmoregulation. What I will say is that moving fish from soft acid water to hard alkaline water or vice versa is even worse, but pH changes alone under same hardness conditions are in my opinion very detrimental.

    I have been using Easylife Easy Carbo on my newly set up 540 l tank with very good results (see separate thread). I have used it at half strength and even then, it works very well.

    Thanks for asking these highly relevant questions as this makes the whole issue clearer to others as well,

    Kind regards,

    Dirk
     
  5. Dirk B
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    Dirk B Aquascaper

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    Hi Alwyn,

    To address your comments:

    You CAN achieve even better growth with CO2 fertilization, about that there is no doubt, so if you have invested heavily, all is not lost. What I am saying is that I am not convinced that the expense of such a setup is fully justified if you come from a soft water area.

    I have a laboratory style pH meter, which for me for keeping fish in soft water, I will not be without. When you have soft water, it is fantastic because you can keep all sorts of beautiful soft water fish, but management of soft water is problematic as it can change its pH so rapidly, but managing this without an accurate pH meter is a disaster. The trick is to keep the conditions optimal with a minimum of water management, something that comes with accurate tools and experience.....

    Kind regards,

    Dirk
     
  6. dart
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    dart Green fingers

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    Fantastic, thanks for clearing that up for me!

    I?ve seen the table interpreted this way once before but the causality of the argument was rather flawed: They essentially argued that adding CO2 lowers the pH and thus lowering the pH would increase the CO2. Obviously that can?t be assumed

    The reaction H+ + HCO3- ↔ H2CO3 ↔ CO2 + H2O does give a cause for the increase in CO2 so I think I get it. BTW I did a quick search on the web and the earth?s atmosphere contains about 383 ppm CO2, so I guess that means that osmosis will easily keep the water?s CO2 levels as high as the pH/KH table indicates (ignoring agitation etc).


    I do have some follow up questions, of course ;-)

    1) Will a drop checker show quantities CO2 added by Excel or Easy Carbo? I.e. I would think that comes down to how plants absorb dissolved organic carbon. I.e. is it absorbed as some complex molecule or does it break down into ?simple? dissolved CO2 and get absorbed that way?


    2) Are there any guidelines for running a medium-tech tank (dosing + excel but no CO2 injection)?

    2.a) What lighting level should we be aiming for? 2/3 watts per gallon?

    2.b) How much macros should I dose? Tom Barr says that plants grown in a medium-tech tank grow at about 5-10 times slower than high tech tanks. So could simply lower my EI dosing schedule by 5 time, right?

    2.c) I assume the same applies to micros?

    2.d) Should we expect an problems converting from high-tech to medium-tech. There is still a drop in CO2 so I assume I can expect algae problems while the plants adapt (producing Rubisco etc)?

    2.e) I?d assume that doing a water change once every month or even once every second month would be fine for a medium tech tank?



    Thanks again for all the information; this has to be the most interesting thread I?ve seen on any aquarium forum so far :)


    PS. sorry for hogging the thread. Come Monday I'll be a father, so getting all this covered now while I still have some time :)
     
  7. Dirk B
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    Dirk B Aquascaper

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    Hi again Andre.

    I am not sure if I understand you here as there is a circularity in your argument. Adding CO2 will lower the pH, but the lowering of the pH does not result in the increased CO2, it is the added CO2 that increases the CO2 and the pH lowering is the result of adding the CO2.

    Yes, and thanks to the effect man is having, the atmosphere's CO2 is rising to levels never seen before, and our plants should grow better than ever before. Osmosis will keep the levels of CO2 as high as possible but it would be necessary to have the surface area sufficiently large. This is where CO2 fertilization does force the issue in that CO2 addition is more efficient than plain osmosis.

    To get to your questions:

    No, the drop checker will not show the CO2 added by Excel or Easy Carbo, as a matter of fact I am sure that this is not added in the form of free CO2 but as organic carbon. It does not break down to CO2 as a result of which it adds carbon completely independently of the whole CO2 issue.

    All of the above questions are linked and are dependent on the amount of light that you illuminate your tank with, the more light, the more macronutrients and the more micronutrients the plants will require and you must dose.

    Again this is entirely dependent on your light, but also on your fish load.

    Thanks for the compliment!

    Ja boet, after this the CO2 in the aquarium is going to be of lesser importance, from now on it is going to be how many hours of sleep can I get, and how many nappies must I change. But don't let me sound too negative, having children is very gratifying, I have three, but they are way past the nappies stage....

    Kind regards and all of the best for Monday,

    Dirk
     
  8. Gertjc
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    Gertjc Algae harvester

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    Good luck with your baby CO2...I mean "AND"
     
  9. James1
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    James1 Noob

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    How do you convert your hardness values (KH) into parts per million? Which wat my test kit gives.
    Thanks

    James
     
  10. dart
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    dart Green fingers

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  11. James1
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    James1 Noob

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    Thanks.
    I just got confused. I have heard of german degrees and some other degrees of hardness.

    James
     
  12. Andre
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    Andre Green fingers

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    Hi there

    This is a very interesting topic, and unfortunately I have not had time to read it carefully. However, I feel that I must point out that the pH, KH, and CO2 have a fixed relationship as long as carbonate is the only buffer present (no phosphate buffers etc) so I do not know if we can assume that adding an acid to lower the pH will automatically increase the CO2 levels in the tank (except if its carbonic acid I guess :) ).
     
  13. Dirk B
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    Dirk B Aquascaper

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    Hi Andre,

    Good to hear from you, I assume you are in the wilds of Cameroon already...

    The point you make is completely correct and important, thanks. In other words if you would add say phosphoric acid to reduce the pH, the effect of the phosphate addition would result in a buffering effect, which in turn would influence the KH and CO2. My recommendation would be to reduce the pH with hydrochloric acid, HCl, which does not have a buffering effect, and not to use phosphoric acid for pH reduction.

    Kind regards,

    Dirk
     
  14. Andre
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    Andre Green fingers

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    Hi Dirk

    You are right, I am in Cameroon now. Thank you for the good advice that you gave me before I left, as Kenyan Airways also managed to misplace my lugguage and at least I had extra clothes and toiletries in my hand lugguage. :)
     
  15. dart
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    dart Green fingers

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    Hi Again,
    Not that I don?t trust you :) but I?ve tried to check this method using a drop checker and bucket of water.
    This is what I did

    • 1.
      • Filled a bucket with tap water (pH 7.8, KH 4)
      • Added drop checker overnight
      • CO2 was very low as expected
      [/li]

    • 2.
      • Added HCl, pH of water now approx 6
      • Left overnight
      • Drop checker showed that CO2 levels were now about 25-30 ppm. Pretty impressive!
      [/li]


    • 3. I was thrilled at this point, however I then...
      • Added an airstone overnight
      • Drop checker showed that CO2 was very low as expected
      • Removed airstone
      • Left for two days
      • Drop checker showed that CO2 levels never increased significantly.
      [/li]

    I checked the KH and pH of the water and left it another day and still no CO2 increased. Can you think of any reason that I?m getting this result?
    Thanks
     
  16. Dirk B
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    Dirk B Aquascaper

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    Hi Dart,

    Thanks for your experiments. I must be quite honest in saying that I do not know how the Easy Carbo works and I think it is going to be very difficult to find out as well as this will be a carefully kept trade secret. However, it is not impossible that it releases the CO2 at lower pH and over a period of 24 hours as you observe. You must also remember that the manufacturers do advise that one should add the mix every day, so they are also not claiming that it is a stable reserve. What it does do, as you have now measured, is that it does add significant CO2 amounts but without the need for expensive CO2 injecting apparatus and I think that is the important take home message. With a daily addition of Easy Carbo, your plants should be well looked after.

    Kind regards,

    Dirk
     
  17. dart
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    dart Green fingers

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    Hi Dirk,

    Thanks for the reply but I dont think I was clear. I was not using easy carbo or excel, rather I was just using plain water and trying to see how adding HCl would affect the CO2 levels.

    The strange result I got was that the CO2 levels did not seem to pick up again even though I had a low pH (6) and soft water (KH 4).

    Does that make sense or am I still missing something?

    Thanks again
     
  18. Dirk B
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    Dirk B Aquascaper

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    Hi Dart,

    Yes, that does change the interpretation of your result for obvious reasons. A possible explanation for the low CO2 levels may be that surface area of your bucket is relatively low, but after two days this should not make such a major difference, so not entirely clear to me. What I can say is that in a tank setup such as I run, I do keep a considerable amount of fish and this I do this without aeration so they do contribute CO2. Perhaps not an entirely convincing argument, I will admit, but it works for me. Maybe I should try to do some measurements with this drop checker. I have never used such a thing. Where can one buy one?

    Kind regards,

    Dirk
     
  19. R.C.
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  20. Cameron
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    Andre

    You cannot add buffers to your water and then use the pH/KH measurements to determine CO2 levels, the readings are not necessarily reflecting CO2 content.

    The water straight out of your tap will naturally be rich in CO2. But put it in a bucket with a bubbler for a few hours and the water will naturally balance itself to the atmosphere, raising pH and obviously oxygen content.
     

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