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Alternative method of intepreting CO2/KH relationship to measure CO2 concentration?

Discussion in 'Advanced Strategies and Fertilization' started by cheeboonyong, Nov 9, 2005.

  1. cheeboonyong

    cheeboonyong Junior Poster

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    Hi,

    I understand the CO2 and KH relationship well enough to read it off the charts.

    And I read this off the "Chuck's Planted Aquarium Pages" (Chuck, if you're reading this, my apologies for not seeking your copyright beforehand)

    Chuck's website

    This calculator (and the chart based on this formula) will only work if your water is carbonate buffered. If your water contains high levels of phosphates, it will alter your water properties, and invalidate these CO2 calculations.

    Of course, I assume that a 1-2 ppm of PO4 should not be an issue if we follow the EI.

    In addition, I suspect there are other sources of acid, e.g., tannic acid from decaying leaves (?), which may distort the pH reading, thus giving us a false impression of the actual level of CO2. Just to state, my tank is 2 years old and mature.

    I have problems with my CO2 reading because I simply don't believe it. My kH is about 9 and my pH reading is 6.7-6.8 (using a pH meter of 0.1 accuracy). This shows a reading of about 42-53 ppm, which is more than adequate. However, I'm still getting BBA outbreaks, which I suspect is due to inadquate CO2. I suspect my water is tainted by some substances (acids?) which is distorting my pH reading.

    However, I came by another paragraph by Chuck, which gave me an idea.

    [Myth: A Low KH results in a larger pH swing when adding CO2.
    Many people are under the mistaken impression that a low KH results in large pH swings when adding CO2, while raising the KH will result in smaller pH swings. This is not the case. The KH will move the start and end pH values, but the pH swing will be the same for a given level of CO2. You can see this in the chart below, or using the calculation:

    Case 1: Assume a KH of 15 degrees, and a starting CO2 level of 4.5ppm, which would result in a pH of 8.0. If we then add CO2, to increase the CO2 level to 28ppm, that would drop the pH down to 7.2, for a pH shift of .8.

    Case 2: Assume a KH of just 1.5 degrees, and a starting CO2 level of 4.5ppm, which would result in a pH of 7.0. If we then add CO2, to increase the CO2 level to 28ppm, that would drop the pH down to 6.2, for a pH shift of .8, the exact same as in case 1. ]

    Therefore, assuming the following:

    kH = 9
    Normal CO2 ppm without CO2 injection at 29 degrees C = 4.5 ppm should give me a reading of pH = 7.8
    To hit target of 30ppm, the pH should be 6.9.
    Hence, the pH swing should be 0.9 - 1.0.

    So, even if my beginning starting pH value is NOT 7.9, all I need to do is to observe my starting pH value and aim for a corresponding reduction of pH 0.9 - 1.0

    I did an experiment by taking a small tub of my tank water and left it in lying around for 24 hours. This should reduce the CO2 to 4.5 ppm (in equilibrium with the atmosphere, I assume). I then took the reading, which showed that my starting pH = 7.5 (NOT 7.8 as predicted, based on kH 9). I do not know what is causing my starting pH value to be lower, maybe unknown acids from decay?

    In any case, my pH measurement after CO2 injection is 6.7 - 6.8, which is only a reduction of about 0.7 - 0.8, not the 0.9 - 1.0, which suggests that my CO2 is not at the desired 30 ppm, hence giving me the BBA.

    Is there something wrong with the way I'm analysing the data? Thanks for reading.

    Cheers
    Boon Yong
     
  2. cheeboonyong

    cheeboonyong Junior Poster

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    Re: Alternative method of intepreting CO2/KH relationship to measure CO2 concentration?

    Just to add, I was using the ADA aquasoil which reduces the pH. However, since the gravel is already 2 years old, I was assuming that whatever was causing the pH distortion has long been leeched out and exhausted. I could be wrong of course

    Boon Yong
     
  3. PeterGwee

    PeterGwee Lifetime Charter Member
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    Re: Alternative method of intepreting CO2/KH relationship to measure CO2 concentration?

    Consider the new misting CO2 method. Change the flow pattern such that all the plants gets the CO2 mist evenly. Add more CO2 slowly till the BBA stops growing. Make sure you have some surface motion, not alot though.

    Regards
    Peter Gwee
     
  4. Tom Wood

    Tom Wood Guru Class Expert

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    Re: Alternative method of intepreting CO2/KH relationship to measure CO2 concentratio

    I've stopped relying on the pH/KH charts because they just don't work in my real-world setup. I use monosodium phosphate for PO4, so the chart is immediately thrown off. It's a shame there isn't a way to adjust the chart for non-carbonate buffers. :gw

    TW
     
  5. PK1

    PK1 Guru Class Expert

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    Re: Alternative method of intepreting CO2/KH relationship to measure CO2 concentration?

    My KH is 5 and I used to have a lot of algae problems when I maintained the tank pH at 6.6. I kept pushing the pH down and most of the problems went away at 6.2-6.3 and I get a lot of pearling at that level. Fish are fine.

    I left the tank water in a glass for 24 hours and read a pH of 7.6 24 hours later.

    I put the charts away.
     
  6. Tom Barr

    Tom Barr Founder
    Staff Member Administrator Social Group Admin

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    Re: Alternative method of intepreting CO2/KH relationship to measure CO2 concentratio

    There are methjods to adjust the CO2.
    Use the plants and algae and of course the fish.

    CO2 measurement is critical.

    I think much like the problems testing with say NO3 kits etc, there is a better way around it without using the kits.

    I'm not ready to abandon the pH/KH measurements, it's going to only skew things lower CO2 ppm wise, so we acheive the 30ppm, then adjust till the tank no longer has a positive impact from SLOWLY adding more and then observing thereafter.

    This is how I found 30ppm to begin with and started suggesting this.
    If you look back I suggested 20-25ppm, then 20-30ppm, now 30ppm.

    A similar pattern exist for NO3 and PO4 dosing, we have gone progressively richer.

    This bothered some folks that felt the nutrients are "too high", but lacked evidence to show that is was "bad". So they dosed daily and try to keep things at a low steady state, which is possible but tough for most folks. Other's pursued a richer sub and a little less water column.

    CO2 will make everything go wrong unless it's in good shape. A little EI will address the issue of nutrients, from there it's all CO2.

    If you have a substrate that is 1-2 years old, vacuum it. Deep vacuum it.
    Every 1-2 years this should be done. You need to remove the mulm before it starts becoming sour in the substrate.

    Mulm is fine to add in the beginning phase to seed things, but later, much like a clogged filter, too much is very bad.

    So you need to uproot and vacuum every 1-2 years.
    Heat Cables, special mircle substrates are not going to remove, nor cycle and breakdown the mulm for you.

    You need to uproot/and/or vacuum.

    No other way around that.

    It's also just a basic thing to remove any sources that are not present in a fairly new tank and vice versus, adding a little mulm to a new tank starts it up right away.



    Regards,
    Tom Barr
     
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