# Ph Kh and CO2 relation



## Jaap (18 Nov 2014)

Hi

I had a talk with Tom Barr last night and he suggested that a pH drop of 1 unit adds approximately 30ppm of CO2 regardles of kH. 

Is this something we should consider? 

What I knew up to now is that if I had a pH drop of something less than a unit but I had a high kH eg 9kH then the CO2 was at disarable levels.

Thanks


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## ian_m (18 Nov 2014)

Yes, you just need a pH meter. Measure before CO2 on and ensure enough CO2 for a drop of 1pH at lights on.

There is a table below, showing that at various dKH values a drop of 1pH gives about 30ppm CO2.
http://tropica.com/en/guide/make-your-aquarium-a-success/fertiliser-and-co2/


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## Marcel G (18 Nov 2014)

Behold, and do yourself and conclusion:

pH 7.7, alkalinity = 2°dKH, temperature 25°C => 1.2 mg/L CO2
pH 6.7, alkalinity = 2°dKH, temperature 25°C => 12.3 mg/L CO2
Difference: ~11 mg/L CO2

pH 7.7, alkalinity = 5°dKH, temperature 25°C => 3.0 mg/L CO2
pH 6.7, alkalinity = 5°dKH, temperature 25°C => 30.6 mg/L CO2
Difference: ~28 mg/L CO2

pH 7.7, alkalinity = 10°dKH, temperature 25°C => 6.1 mg/L CO2
pH 6.7, alkalinity = 10°dKH, temperature 25°C => 61.3 mg/L CO2
Difference: ~55 mg/L CO2

pH 7.7, alkalinity = 15°dKH, temperature 25°C => 9.1 mg/L CO2
pH 6.7, alkalinity = 15°dKH, temperature 25°C => 91.9 mg/L CO2
Difference: ~83 mg/L CO2

xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

pH 7.4, alkalinity = 2°dKH, temperature 25°C => 2.4 mg/L CO2
pH 6.4, alkalinity = 2°dKH, temperature 25°C => 24.5 mg/L CO2
Difference: ~22 mg/L CO2

pH 7.4, alkalinity = 5°dKH, temperature 25°C => 6.1 mg/L CO2
pH 6.4, alkalinity = 5°dKH, temperature 25°C => 61.2 mg/L CO2
Difference: ~55 mg/L CO2

pH 7.4, alkalinity = 10°dKH, temperature 25°C => 12.2 mg/L CO2
pH 6.4, alkalinity = 10°dKH, temperature 25°C => 122.3 mg/L CO2
Difference: ~110 mg/L CO2

... or use my CO2 calculator: www.prirodni-akvarium.cz/en/index.php?id=en_chemistry2


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## Jaap (18 Nov 2014)

ardjuna said:


> Behold, and do yourself and conclusion:
> 
> pH 7.7, alkalinity = 2°dKH, temperature 25°C => 1.2 mg/L CO2
> pH 6.7, alkalinity = 2°dKH, temperature 25°C => 12.3 mg/L CO2
> ...


How do you measure the co2? I mean you measure the alkalinity with a kh meter and the ph with a ph meter but do you have a co2 meter? 

Thanks


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## Jaap (18 Nov 2014)

ian_m said:


> Yes, you just need a pH meter. Measure before CO2 on and ensure enough CO2 for a drop of 1pH at lights on.
> 
> There is a table below, showing that at various dKH values a drop of 1pH gives about 30ppm CO2.
> http://tropica.com/en/guide/make-your-aquarium-a-success/fertiliser-and-co2/


So kh is related to ph in regard to co2? I am trying to understand why Tom Barr said 1 unit drop of ph is 30ppm co2 regardless of kh...


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## ian_m (18 Nov 2014)

Jaap said:


> So kh is related to ph in regard to co2?


Yes see the table I posted a link to.


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## Paulo Soares (18 Nov 2014)

But i have a pertinent and important question: 

*What are the consequences of this abrupt drop of PH and a suddenly raise of the CO2 in the fish and even in the plants?*

*Is not more reliable to inject Co2 in a stable or minor leverl so that as it is being injected is being consumed from the plants?
*
Best regards


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## Marcel G (18 Nov 2014)

Jaap said:


> How do you measure the co2? I mean you measure the alkalinity with a kh meter and the ph with a ph meter but do you have a co2 meter?


No, the above values are based just on KH and pH measurements and relationships. Still I believe that in most cases it should be quite close to the reality (but of course, in some cases it could be wrong).


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## Jaap (18 Nov 2014)

So how do we really know kh and ph are related if we cannot independantly measure co2? Maybe Tom Barr ir right and maybe not...how can we know?


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## Edvet (18 Nov 2014)

KH doesn't influence the amount of CO2 in the water, it influences the associated pH drop ( buffers it). So little KH, means little buffering -> larger pH drop, lots of KH, large buffering capacity, -> small pH drop.


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## Jose (18 Nov 2014)

Tom is right yes. Doesnt matter your KH, if you lower your ph by one unit you will have around 30 ppm of CO2 in your tank. This is very simple and no need to overthink it. Look at the ph-kh table and you should be able to see that at different KHs your ph will vary the same as to add 30 ppm CO2.

By the way you also dont need more CO2 added in a tank with higher KH compared to a lower KH one. 
Basically you can forget about KH with regards to CO2.


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## pepedopolous (18 Nov 2014)

I think Mr Barr is just offering simple guidelines to try and make our hobby easier 

If you have a high KH then the high buffering capacity means you *don't need* a 1 pH unit decrease to get 30ppm CO2. You're guaranteed at least 30ppm if you get a 1pH drop, but your fish might not appreciate it!

ardjuna, it's a pity your table doesn't work for Prague water!


P


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## Jose (18 Nov 2014)

pepedopolous said:


> If you have a high KH then the high buffering capacity means you don't need a 1 pH unit decrease to get 30ppm CO2.



This is just not true. But everyone seems to think it is no matter what you have in your water (eg phosphate buffers etc).


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## Jose (18 Nov 2014)

http://www.barrreport.com/showthread.php/11066-Different-CO2-forms-under-low-vs-high-pH


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## pepedopolous (18 Nov 2014)

Hi Jose, I read the link you provided and still not sure. They seem to be talking about whether CO2 addition changes KH, and conclude that it doesn't.

I understand that KH/PH tables always _underestimate_ CO2 concentration as pH is affected by other compounds in aquarium water. The only case where the tables apply accurately is inside a drop checker because we control the content of the drop checker solution and keep the KH at 4. The CO2 crosses the air gap to the drop checker solution and the pH decreases giving a colour change.

However in an aquarium, no matter the KH of the water and the pH before CO2 is added, when we add CO2 the pH decrease we measure during that time is simply a result of the added CO2.  Other compounds in the water are highly unlikely to have a significant effect during this time.

Where you and I seem to disagree is that I think that the KH is relevant because if you add X amount of CO2 to low KH water you will see a bigger decrease in pH than in high KH water (buffering).

So I think you can add CO2 to aquarium water, making the concentration 30ppm and yet for high KH water the pH may not drop by a full unit.

Cheers,

P


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## Jose (18 Nov 2014)

Sorry about that. Ill look for the threads on the matter when I have some time but have a read around the barr report its all there.

"However in an aquarium, no matter the KH of the water and the pH before CO2 is added, when we add CO2 the pH decrease we measure during that time is simply a result of the added CO2. Other compounds in the water are highly unlikely to have a significant effect during this time."

This is totally true.

"So I think you can add CO2 to aquarium water, making the concentration 30ppm and yet for high KH water the pH may not drop by a full unit."

This last bit is where I disagree.

Lets do something. Look at the ph-kh table. Take two tanks as examples; One with KH 1 and the other one with KH 10. Suppose you have 8 ppm of CO2 at the starting equilibrium for both when you are not adding CO2 and write down their equilibrium ph. Then we start adding CO2 until we get to 30 ppm. At what ph are we now? write them down. Now do the first ph for each tank minus the second one after we add CO2. How much is the jump in ph?.


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## AverageWhiteBloke (19 Nov 2014)

ardjuna said:


> Behold, and do yourself and conclusion:
> 
> pH 7.7, alkalinity = 2°dKH, temperature 25°C => 1.2 mg/L CO2
> pH 6.7, alkalinity = 2°dKH, temperature 25°C => 12.3 mg/L CO2
> ...



Wow never came across this before! Bit confused over the salinity part, how do you determine this?


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## Marcel G (19 Nov 2014)

Jose said:


> http://www.barrreport.com/showthread.php/11066-Different-CO2-forms-under-low-vs-high-pH


Jose, first I want to tell that "kwisatz" from barrreport.com, and "ardjuna" from ukaps.org is one and the same person ... me. So it was my thread on barrreport.com.

Secondly, it seems to me that you do a wrong assumption if you say that you can have 8 mg/L of CO2 before you start adding CO2 into your tank. If you don´t have some really bad tank with a lot of organics, then you can have around 3-5 mg/L CO2 at equilibrium with air + a few fish waste. Still, I don´t understand your logic. Please, can you give me some example of your counts? If I have pH 6.7 at KH 1, then I´ll have (theoretically) 6 mg/L of CO2. If I have pH 6.7 at KH 10, then I have (theoretically) 61 mg/L of CO2 ... but in reality I wont have 61 mg/L CO2, but I´ll have around 3-5 mg/L ... because that´s the equilibrium level before you add any artificial CO2 into your tank. The 61 mg/L CO2 is just theoretical level (in case you add artificial CO2). The same apply for pH 7.7 at KH 1 = 0.6 mg/L CO2, vs. pH 7.7 at KH 10 = 6 mg/L CO2.


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## Marcel G (19 Nov 2014)

AverageWhiteBloke said:


> Bit confused over the salinity part, how do you determine this?


You can estimate the salinity level based on conductivity: each 100 µS/cm (conductivity) ≈ 0.05 ‰ salinity.


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## AverageWhiteBloke (19 Nov 2014)

> You can estimate the salinity level based on conductivity: each 100 µS/cm (conductivity) ≈ 0.05 ‰ salinity.


Ah right, and can this be determined using a TDS meter? I've came across a few conversion sites but no exactly sure which to convert.

Came Across this....
Electrical Conductivity units are expressed in micro-siemens/cm at 25°C (µS/cm).
1000 EC = 1000 µS/cm = 640 ppm (parts per million)

So 0.64 ppm per 1µS/cm?

Probably got that all wrong


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## dw1305 (19 Nov 2014)

Hi all, 





AverageWhiteBloke said:


> Ah right, and can this be determined using a TDS meter? I've came across a few conversion sites but no exactly sure which to convert.
> Came Across this....
> Electrical Conductivity units are expressed in micro-siemens/cm at 25°C (µS/cm).
> 1000 EC = 1000 µS/cm = 640 ppm (parts per million)
> So 0.64 ppm per 1µS/cm?


 That is the normal conversion factor.  

Realistically you can ignore the salinity in fresh water,  it becomes relevant in full salinity sea water, but that is about 53,000 microS.  





ardjuna said:


> or use my CO2 calculator: <"www.prirodni-akvarium.cz/en/index.php?id=en_chemistry2">


 I'm not an "additional CO2" user, but that is a very useful chart.

cheers Darrel


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## Jose (19 Nov 2014)

ardjuna said:


> Secondly, it seems to me that you do a wrong assumption if you say that you can have 8 mg/L of CO2 before you start adding CO2 into your tank. If you don´t have some really bad tank with a lot of organics, then you can have around 3-5 mg/L CO2 at equilibrium with air + a few fish waste. Still, I don´t understand your logic. Please, can you give me some example of your counts?
> 
> 
> ... but in reality I wont have 61 mg/L CO2, but I´ll have around 3-5 mg/L ... because that´s the equilibrium level before you add any artificial CO2 into your tank. The 61 mg/L CO2 is just theoretical level (in case you add artificial CO2). The same apply for pH 7.7 at KH 1 = 0.6 mg/L CO2, vs. pH 7.7 at KH 10 = 6 mg/L CO2.



You seem to be misinterpreting the table.

The 8ppm of CO2 at the beginning was just to make the point and you shouldnt get hung up on that, its irrelevant to what I was trying to show you.



ardjuna said:


> If I have pH 6.7 at KH 1, then I´ll have (theoretically) 6 mg/L of CO2. If I have pH 6.7 at KH 10, then I have (theoretically) 61 mg/L of CO2 ... but in reality I wont have 61 mg/L CO2, but I´ll have around 3-5 mg/L


This is where you are misreading the table. You cant go from a water of ph 6.7 and kh 1 to a water with same ph and 10 kh, in other words you are not changing the KH in your tank (with carbonates) and if you are your ph will change as well. See what I mean?

So, when you read the ph-kh table you have to stay in the same KH line. The only time you go from one KH line to another is to compare different tanks/waters. And this is what I did before to make the point that: to get 30 ppm CO2 your ph has to change 1 unit, no matter what your KH is.


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## Jose (19 Nov 2014)

ardjuna said:


> Jose, first I want to tell that "kwisatz" from barrreport.com, and "ardjuna" from ukaps.org is one and the same person ... me. So it was my thread on barrreport.com.


Then this should be clear enough by now.


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## Jose (19 Nov 2014)

ardjuna said:


> Please, can you give me some example of your counts?


Ill do another example:

Tank 1: KH5, pH 7.7 at equilibrium and 3ppm CO2 at equilibrium.
Tank 1 with CO2 addition: KH5, ph 6.7 and 30 ppm CO2

Tank 2: KH 20, pH 8.3 at equilibrium and 3ppm CO2
Tank 2 after adding CO2: KH 20, ph 7.3 and 30 ppm CO2

All these is taken from the ph-kh table on the net.

Thats it two tanks with different KH waters but keep in mind that if you change your pH by adding something to the water like an acid or base then your KH will change as well.

Notice though that I am not saying 30 ppm is the CO2 level for everyone. I personally dont go even near.


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## AverageWhiteBloke (19 Nov 2014)

> Realistically you can ignore the salinity in fresh water, it becomes relevant in full salinity sea water, but that is about 53,000 microS.
> ardjuna said: ↑
> or use my CO2 calculator: <"www.prirodni-akvarium.cz/en/index.php?id=en_chemistry2">
> I'm not an "additional CO2" user, but that is a very useful chart.
> ...



I sort of guessed that when I changed the salinity to what my tank TDS would be on the chart and it didn't really affect the out come other than some insignificant numbers. Totally agree that it's a great chart though if you're into that sort of thing. I just try and understand this stuff because I have an enquiring mind. Fair to say I won't be going home tonight, breaking out the tests and making any adjustments though. 

One of the pitfalls of using data as accurate as this is with it's decimal point answers is that I guess most of us mere mortals just have a liquid KH test which will be slightly out sending the rest of the figures with it. It's sort of built on a shaky foundation. I guess the chart at the bottom is as good as anything for what we're trying to achieve.

Over the last year or so I've found alternate ways of adjusting co2 rather than reaching for the needle valve. I try and keep my injection at a constant. What I do is try and anticipate what the tank will do. For instance when my plant mass is quite high I'll fill the tank up a bit more to reduce surface agitation from the lily pipe and vice versa when I've had a good clip out I'll keep it low to let some gas out. The best way I have found of controlling co2 is by reducing and increasing the plants which seems to benefit the plants.

This is where the drop checker really comes in handy. As slow as it is and not very accurate depending on where it's positioned I sort of watch it and when I see it darkening in colour as the mass builds up it's time for a clip out.  The needle valve is the last resort for me once I have a baseline level. Coincidentally using the KH, PH method using a liquid test and calibrated pen it shows that I have about 67ppm co2 at lights on time  Observing the tank and fish as well as the drop checker tells me that this is probably quite a high estimate.


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## Marcel G (19 Nov 2014)

Jose said:


> You seem to be misinterpreting the table...to get 30 ppm CO2 your ph has to change 1 unit, no matter what your KH is.


This is wrong assumption! I don't know how you get to this conclution? Please, look at my page: http://www.prirodni-akvarium.cz/index.php?id=co2ph, where you can find real pH and KH measurements in my tanks.
To list a few:
1) Tank #1: pH 7.2 at night, pH 6.4 at day (during photosynthesis), KH 4.5 => 8.7 mg/L CO2 at night, and 55 mg/L CO2 at day
2) Tank #2: pH 6.9 at night, pH 5.9 at day, KH 2.5 => 10 mg/L CO2 at night, and 77 mg/L CO2 at day
3) Tank #3: pH 7.2 at night, pH 6.2 at day, KH 2.5 => 5 mg/L CO2 at night, and 49 mg/L CO2 at day
4) Tank #4: pH 6.8 at night, pH 6.1 at day, KH 4.5 => 22 mg/L CO2 at night, and 110 mg/L CO2 at day
What do you see in the above real-life values?
You can see the following:
1) pH difference: 0.8 points, CO2 difference: 46 mg/L
2) pH difference: 1.0 points, CO2 difference: 67 mg/L
3) pH difference: 1.0 points, CO2 difference: 44 mg/L
4) pH difference: 0.7 points, CO2 difference: 88 mg/L
Do you still claim (based on these facts) that each 1 point difference in pH mean 30 mg/L CO2?


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## Jose (19 Nov 2014)

You would be the only one who gets 88 mg/l of CO2 with a 0.7 ph difference then. Ive never seen this happen before.

How do you meassure CO2?
How do you meassure pH and if you use a pH pen do you calibrate it? Also when you use a ph probe the current from the apliances in the tank might effect it(¿?). I havent got experience with this. I know there has to be enough flow also for the probe.
Are you adding something to the wáter?
How do you meassure KH?

There must be something else going on. Can you explain all the process you go through to get to those results please? Ill have a read from google translator from your page tomorrow if I get the chance.

If this is normal can somebody like ceg chime in please.


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## Jose (19 Nov 2014)

By the way the pH jump I mean is from the equilibrium with air(around 2/3ppm of CO2) to 30 ppm of CO2 not from 10, 5(not sure if this is your equilibrium) or 22 to whichever value. The jump from 20 ppm of CO2 to 50 is not necessarily going to generate the same pH swing as going from 2 to 32 ppm CO2, yet its hte same jump difference in CO2.


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## Marcel G (20 Nov 2014)

Jose said:


> How do you meassure CO2? How do you meassure pH and if you use a pH pen do you calibrate it? ...
> How do you meassure KH?


If you read the article you would know that I have used Extech SDL100 pH/ORP meter with datalogger. I used lab grade electrode with this meter, and yes, I did calibrate it using calibration solution of pH 4.0, pH 7.0, and pH 10.0. Also I kept the electrode in my tanks for a whole week!, so because of the datalogger function, the probe did the measuments in 10 minutes intervals, so the values should be quite reliable. As to the KH, I used JBL and Sera hobby tests (each with double water amount to get more precise numbers), and two times I did used a lab analysis also. As to the pH jump, your assumption that each 1 point difference in pH mean 30 mg/L CO2 is just wrong, and I did demostrated it by my practical (and also theoretical) tests. If you still insist on that, then I cann't do anything more with it. If you can live with it, then just ignore the facts.

And I would like to tell you one more thing: I hope that you did ask the same things (i.e. to explain in details his methodology) Tom Barr also. Otherwise I just don't understand your point in believing some person who says something (without any proof), while doubting what another person sais even if he gives you all the data. If you are so strong in the assumption that each 1 point difference in pH mean 30 mg/L CO2, then you must have some strong data to support your statement.


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## Jaap (20 Nov 2014)

ardjuna said:


> If you read the article you would know that I have used Extech SDL100 pH/ORP meter with datalogger. I used lab grade electrode with this meter, and yes, I did calibrate it using calibration solution of pH 4.0, pH 7.0, and pH 10.0. Also I kept the electrode in my tanks for a whole week!, so because of the datalogger function, the probe did the measuments in 10 minutes intervals, so the values should be quite reliable. As to the KH, I used JBL and Sera hobby tests (each with double water amount to get more precise numbers), and two times I did used a lab analysis also. As to the pH jump, your assumption that each 1 point difference in pH mean 30 mg/L CO2 is just wrong, and I did demostrated it by my practical (and also theoretical) tests. If you still insist on that, then I cann't do anything more with it. If you can live with it, then just ignore the facts.
> 
> And I would like to tell you one more thing: I hope that you did ask the same things (i.e. to explain in details his methodology) Tom Barr also. Otherwise I just don't understand your point in believing some person who says something (without any proof), while doubting what another person sais even if he gives you all the data. If you are so strong in the assumption that each 1 point difference in pH mean 30 mg/L CO2, then you must have some strong data to support your statement.


According to your table on page one, I have 55mg/l of co2 added. Is this lethal to fish? If this is lethal then its either wrong because my fish are still alive or the surface agitation provides o2 and fish are not digased...if its not lethal is it considered a good amount of co2?


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## Edvet (20 Nov 2014)

You can't use a table/measurement in another tank for your own tank, to many variables, that's why a pH profile of a tank gives better understanding of what's going on in that tank than actual values ( probe quality, calibration, other acids in the tank). In normal/usual KH values Barr's assumption will be approximatly correct (1 point drop will be app 30 mg/l and should be sufficient for most demanding plants). At the extreme ends in can't be, that's simple Chemistry.
Jaap I think it's highly unlikely you'll be able to add that amount, and the fish show that. Tables will always overestimate the amount of CO2, that's where a DC can help


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## Marcel G (20 Nov 2014)

Jaap said:


> According to your table...


First of all, this is not _*my*_ table. These are data calculated according to the scientistific findings (see my website for more info and citations). I suppose these data _*should*_ be correct under common circumstances in most tanks, but the CO2-KH-pH correlation is quite complicated. What most people don't realize is the fact, that this CO2-KH-pH table is based on KH values, but the KH we measure using our KH tests is not the KH used in this table! If we measure KH, then in fact we measure alkalinity not KH. KH denotes ions of calcium and magnesium bound exclusively to bicarbonates, so it's the content of dissolved [calcium and magnesium] bicarbonates. Whereas alkalinity is the content of all kinds of bicarbonates and carbonates (not only calcium and magnesium ones!). So with alkalinity not only HCO3- a CO3--, but also PO4--, SO4-- etc. play some role. So this we should know and use the table with caution. All these basics can be found here (it's in Czech) or elsewhere. So I don't know if you have 55 mg/L CO2 in your tank in reality, as I don't have any CO2 meter to find out the real-life differences between these theoretical tables (based on scientific findings and calculations) and actual CO2 concentration in your/my tank. You can ask Tom Barr if he's willing to make some tests with his CO2 meter to compare the theoretical data with actual measured values, and publish his results together with the methodology. My only concern here was to point out that the assumption that "each 1 point difference in pH mean 30 mg/L CO2" is wrong.

_Few notes to what lethal concentration to fish could mean:_
Eash fish has different tolerance to CO2 levels, so I think it's not easy to say what CO2 concentration is OK. Shrimps should be less tolerant to higher CO2 levels, as they belong among invertebrates which tend to be more sensitive. I myself had (theoretically) around 80 mg/L CO2 in my tank a couple of times, and my fish survived, but some shrimps died. Tom Barr states that he maintains 50-70 mg/L CO2 in his main tank. Still, I don't believe that these high levels are OK in all circumstances and to all critters (especially for fingerlings). Also I don't believe that these levels are needed in first place. But it seems from the experiences of many users, that high CO2 levels can suppress (some) algae ... which is probably the main reason some people encourage others to increase, increase, and increase their CO2 levels as the final cure to all algae problems (high concentration of CO2 could be lethal not only to fish/shrimps, but to algae also!).


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## Jose (20 Nov 2014)

ardjuna said:


> And I would like to tell you one more thing: I hope that you did ask the same things (i.e. to explain in details his methodology) Tom Barr also. Otherwise I just don't understand your point in believing some person who says something (without any proof), while doubting what another person sais even if he gives you all the data. If you are so strong in the assumption that each 1 point difference in pH mean 30 mg/L CO2, then you must have some strong data to support your statement.



You are misinterpreting my words here and Tom Barrs affirmations. This is what ive said before and I stand by it:



Jose said:


> By the way the pH jump I mean is from the equilibrium with air(around 2/3ppm of CO2) to 30 ppm of CO2 not from 10, 5(not sure if this is your equilibrium) or 22 to whichever value. The jump from 20 ppm of CO2 to 50 is not necessarily going to generate the same pH swing as going from 2 to 32 ppm CO2, yet its hte same jump difference in CO2



I am not talking about a change from 20 ppm CO2 to 50 ppm CO2. This jump wouldnt create a 1 pH unit change. What Tom Barr is saying is that a pH change from equilibrium (2 or 3 ppm CO2) to 30 ppm will create a 1 ph unit drop. Notice, from equilibrium, not from your night CO2 levels.

If More people get to the same results as you I would be happy to say you are absolutely right but there doesnt seem to be any evidence for this. Whilst Ive seen the 1 pH unit drop for myself compared to a dropchecker.


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## Jose (20 Nov 2014)

ardjuna said:


> First of all, this is not my table. These are data calculated according to the scientistific findings (see my website for more info and citations). I suppose these data should be correct under common circumstances in most tanks, but the CO2-KH-pH correlation is quite complicated. What most people don't realize is the fact, that this CO2-KH-pH table is based on KH values, but the KH we measure using our KH tests is not the KH used in this table! If we measure KH, then in fact we measure alkalinity not KH. KH denotes ions of calcium and magnesium bound exclusively to bicarbonates, so it's the content of dissolved [calcium and magnesium] bicarbonates. Whereas alkalinity is the content of all kinds of bicarbonates and carbonates (not only calcium and magnesium ones!). So with alkalinity not only HCO3- a CO3--, but also PO4--, SO4-- etc. play some role. So this we should know and use the table with caution. All these basics can be found here (it's in Czech) or elsewhere. So I don't know if you have 55 mg/L CO2 in your tank in reality, as I don't have any CO2 meter to find out the real-life differences between these theoretical tables (based on scientific findings and calculations) and actual CO2 concentration in your/my tank. You can ask Tom Barr if he's willing to make some tests with his CO2 meter to compare the theoretical data with actual measured values, and publish his results together with the methodology. My only concern here was to point out that the assumption that "each 1 point difference in pH mean 30 mg/L CO2" is wrong.



Yes there can be errors in meassuring KH this is why a pH jump is suggested, so that you can forget about KH measuring. This way all you have to do is measure the pH swing because no matter what water you have a 1pH swing will mean approx 30 ppm of CO2 of course if your equilibrium or starting point is water let to rest on a glass for some time.


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## Marcel G (20 Nov 2014)

Jose said:


> Notice, from equilibrium, not from your night CO2 levels.


So, finally I got your point ... sorry to take it so long. 

Still, I'm not convinced that your point is correct. Because your assumption may be true only in case that your CO2 concentration at equilibrium point is around 3 mg/L. But what if your equilibrium concentration would be 10 mg/L CO2? In such a case you kill all the life in your tank with this attitude (= decrease the pH by 1 point to reach optimal CO2 level). One friend of mine measured his CO2 level using dropchecker (which is not something extra precise, but still quite precise compared to other methods) in his non-CO2 tank, and he found out that in this tank the CO2 level at equilibrium state [not at equilibrium state with air only, but at equilibrium state with the fish and waste products also!] is somewhere between 10-15 mg/L. These higher levels will apply especially for the tanks with higher organic load, where the organic matter decomposes to CO2, thus increasing the starting level.

So I have one important question on you: _*How do you measure (find out) your initial CO2 level, if you don't want to use any KH test?*_ Do you want to trust the assumption that in your tank you have around 3-5 mg/L CO2 at equilibrium state? _*What if you have more?*_


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## Jose (20 Nov 2014)

ardjuna said:


> So I have one important question on you: How do you measure (find out) your initial CO2 level, if you don't want to use any KH test? Do you want to trust the assumption that in your tank you have around 3-5 mg/L CO2 at equilibrium state? What if you have more?



You dont need to know your initial CO2 levels.

All you need to do is: Take a glass of water from your tank and let it seat for a day or some hours. Measure ph and then you can try to get that pH-1 unit in your tank. This is if you want 30 ppm. Another thing you can do to try and proof this is wrong is the following:

I hope You run this experiment and update us. All you need to do is measure the pH of the water from your tank in a glass which had time to rest. Then measure your tanks pH and your CO2. Will see if this makes sense comparing it to the pH KH table which should work perfectly fine since you are not considering KH here.


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## Jose (20 Nov 2014)

Also another way you can make sure there is no extra co2 in the water is by shaking it. The CO2 should be degassed and dissolve to equilibrium with air.


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## Marcel G (20 Nov 2014)

Jose said:


> You dont need to know your initial CO2 levels. All you need to do is: Take a glass of water from your tank and let it seat for a day or some hours. Measure ph and then you can try to get that pH-1 unit in your tank. This is if you want 30 ppm.


That's quite _*interesting idea.*_ I like it, and I may try it ... but not now, as right now I'm doing some tests with nutrient consumption.


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## Jose (20 Nov 2014)

Cool. Tell us when we can see the results


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