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Discussion Starter · #1 · (Edited)
There have been several discussions about the inaccuracy of CO2 measurement using KH and PH. It was suggested that we take a tank sample and let it sit overnight to reach equilibrium with the CO2 in the atmosphere. That was assumed to be about 3 ppm. If the PH increased by about 1.0 the CO2 was assumed to be about 30 ppm. This led me to look at the CO2/PH/KH equation some more and reduce it to a difference between equilibrium PH and tank PH, to give the attached chart. Any comments? If this is mathematically correct, it looks like a more accurate way to measure CO2, without worrying about what the KH is.



Edit: My experimentation indicates that the equilibrium ppm for my kitchen is about 4 ppm. This chart has been revised to reflect that.
 

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I absolutely agree that the method you describe is a better way to look at CO2 concentration since it removes many of the variables that can upset the traditional KH/pH/CO2 chart. Organic acids and non-carbonate buffers are always present to some degree in a real aquarium which invalidate the equation the chart was derived from.

Your chart above should be correct if your assumption of 3 ppm of dissolved CO2 at equilibrium is, in fact, true.

CO2 (ppm) = equilibrium CO2 (ppm) X 10 ^ change in pH

I've been looking all over the web looking for reference values for dissolved aqueous CO2 levels at atmospheric equilibrium and so far I haven't found anything outside of aquarium related sites to verify this number.

For anyone that hasn't been following this discussion over the past two or three months, the basic idea is to set some aquarium water aside in a container with a large surface area, or to bubble air through the sample. If you don't aerate the sample, allow 24-48 hours for the pH to stabilize as it comes to equilibrium. The differenece in pH between the degassed sample and the water in your tank comes from the difference in CO2 levels.
 

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guaiac_boy said:
I absolutely agree that the method you describe is a better way to look at CO2 concentration since it removes many of the variables that can upset the traditional KH/pH/CO2 chart. Organic acids and non-carbonate buffers are always present to some degree in a real aquarium which invalidate the equation the chart was derived from.

Your chart above should be correct if your assumption of 3 ppm of dissolved CO2 at equilibrium is, in fact, true.

CO2 (ppm) = equilibrium CO2 (ppm) X 10 ^ change in pH

I've been looking all over the web looking for reference values for dissolved aqueous CO2 levels at atmospheric equilibrium and so far I haven't found anything outside of aquarium related sites to verify this number.

For anyone that hasn't been following this discussion over the past two or three months, the basic idea is to set some aquarium water aside in a container with a large surface area, or to bubble air through the sample. If you don't aerate the sample, allow 24-48 hours for the pH to stabilize as it comes to equilibrium. The differenece in pH between the degassed sample and the water in your tank comes from the difference in CO2 levels.
Well now you're assuming we all have access to accurate pH readings. I use an AP pH test kit and a Red Sea pH test kit. Sometimes they agree, sometimes they are off from the other by 0.2.
 

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banderbe said:
Well now you're assuming we all have access to accurate pH readings. I use an AP pH test kit and a Red Sea pH test kit. Sometimes they agree, sometimes they are off from the other by 0.2.
Yeah, even pH probes can be off by quite a bit - especially if you don't callibrate often or replace them when they wear out. Color comparison test kits can be even harder to get accurate readings out of, not to mention innacuracies in measuring KH.

Like everything else though, two decimal precision is great for labs but a little ridiculous for a hobby. I'm assuming everyone does the best with what they've got. I think even the cheapest pH test kit can distinguish between a 0.4 and 1.0 pH difference. This will at least get you in the ballpark, especially with several readings over time.

Another method that might be the best of all is to slowly crank up the CO2 until the fish look like they're not liking it. Then back off about 0.2 or 0.3 pH units. Go slow and only do it when you're around to watch the fish. As long as the plants are growing and algae isn't, there isn't much point in seeing how far you can push it.
 

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acbaldwin said:
What about other things that affect your pH reading? Wouldn't they lead to a false CO2 reading?
Not at all..... That is the beauty of this method. If you take the water right from the tank and let it degas, the only difference between the two samples will be the concentration of CO2. The pH of both will certainly be affected by organic acids, tannins, phosphate buffers, etc, but equally so.
 

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Discussion Starter · #7 ·
This method seems to me to have only one unique disadvantage, and that, of course, is the time it takes to get a reading. But, it does appear to cancel out all of the various other errors we run into by measuring KH and PH.

Today an "old timer" who is well known in aquatic plant circles was upset that we would even consider using fish as a test device - torturing them with ever higher CO2 levels until their distress is obvious to us. That did impress me and started me to thinking harder about how else to measure CO2.
 

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hoppycalif said:
There have been several discussions about the inaccuracy of CO2 measurement using KH and PH. It was suggested that we take a tank sample and let it sit overnight to reach equilibrium with the CO2 in the atmosphere. That was assumed to be about 3 ppm. If the PH increased by about 1.0 the CO2 was assumed to be about 30 ppm. This led me to look at the CO2/PH/KH equation some more and reduce it to a difference between equilibrium PH and tank PH, to give the attached chart. Any comments? If this is mathematically correct, it looks like a more accurate way to measure CO2, without worrying about what the KH is.
Here are my test results from a few months ago:

Tap pH - 8.51.
Rested Tap - 7.59.
Tank - 6.16
Tank Aerated - 7.67

That gives me about 95ppm of CO2 according to the chart. I'm thinking this chart may be fairly close. If I add fish without using drip acclimation for several hours, the fish pretty much sink to the bottom of my tank in a matter of minutes.

I will try and run some more tests on my tank water tomorrow if time permits. It would be nice if we could find a way to eliminate one more test kit that new folks will have to purchase!

I can imagine someone being upset by harming the fish, but if caught before they are unconscious, it should be temporary. Once you see them start to get stressed, you can easily back off the CO2 before any damage is done. Even the fish that sank to the bottom of my tank were revived by aerating them in a small volume of water.
 

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One thing to note about pH testers (digital)...

regardless of calibration (accuracy), as long as the precision is fine this will work. Readings can be off by whatever amount, but they'll both be off by the same amount. An inexpensive hand held digital meter should be quite good enough and cheaper than test kits in the long run.
 

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hoppycalif - that particular old timer is also the only person I know of who worked out the equilibrium levels for CO2 from air, with compensation for altitude/barometric pressure. Check out his web site, it has to be there somewhere, or on The Krib, or both.
 

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Hello friends,

Aren't you still bored of this CO2 conversations. Yes it is a must for all planted tanks but measuring the level is very tricky as there are many factors affecting all the variables. So you can not be sure that the level you read from any of the tables is 100% accurate. What I do is always try to keep it as high as possible and the only indicator for me is fish distress.

YILDIRIM
 

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I think that 3 ppm is far from the true value of the equilibrium.
A simple calculation using Henry´s law gives a value of 0,5 ppm of dissolved CO2 in equlibrium with atmosphere.
 

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i agree with YILDIRIM i run my co2 as high as i can get away with as long as you are above 30 ppm u are going to be fine let the fish tell u how high it is lol. everyone gets to hung up on spot on testing.
 

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John S said:
i agree with YILDIRIM i run my co2 as high as i can get away with as long as you are above 30 ppm u are going to be fine let the fish tell u how high it is lol. everyone gets to hung up on spot on testing.
I agree with both of you on the testing. Even with good kits and meters there are some variables, not to mention human error, and no one test will be perfect. I rarely test unless something appears to be wrong but this can be a difficult concept to grasp for those without much experience. When you are new to the hobby, it is nice to have some "numbers" to go by until you gain a bit of experience.

According to the KH/pH chart, my levels are over 200ppm (LaMotte Alkalinity test and Hanna 98129 meter) and if I were to follow the advice at keeping my levels in the 30-40ppm range, I would have massive BBA in my tank. Go ahead and ask how I know this ;)

My fish do not gasp at the 200ppm range (90ppm with Hoppy's method) but new fish succumb very quickly to these levels. I have been increasing the CO2 very slowly for a long time now and it appears that the fish may be able to adapt somewhat.

Raising the CO2 levels can be very dificult for someone new to the hobby to understand. I myself was a victim of this when I first started following the advice of folks to increase my CO2 levels. Go ahead and ask how I know this too ;) I think what Hoppy and the rest of us are trying to arrive at is a simpler way for beginners to determine CO2 levels and not kill off their fish!

If a new method can help only one person get into the hobby and stay here, then I think it is a worth while discussion :)
 

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Kudos MatPat,

Exactly what I wish I could have said. The same logic applies to everything that can sometimes derail those with less experience. I've kept fish long enough to know when they don't look 'right' and I can usually get out of trouble pretty quickly, but all the chemistry behind nutrient levels and CO2 had me baffled for the longest time. Net result = tons of algae & confused about what to do with it.

After getting Lamotte kits and doing some testing for a few weeks I now know exactly what high and low nitrates look like in my tank. It's much easier now to figure out what to do. I also can tell roughly what the iron situation is like just by looking at the leaves after several months of observation.

It's easy to forget that you didn't always know something. It's the same thing with CO2, but the stakes are higher, especially when you don't have experience to fall back on. Make a little mistake = algae attack. Make a big mistake = dead fish.
 

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Discussion Starter · #17 ·
The problem I ran into when I started my pressurized CO2 was that when I had 30 ppm +/- a bit, I felt I was at the max, but I had BBA starting up with a fury. So, I decided it was worth it to me to sacrifice my half dozen fish by going higher to try to stop the BBA. I did so, and the fish were fine. I kept doing so and the fish were still fine. Eventually, by removing most of the BBA and keeping a really high level of CO2, 170ppm per the chart, I stopped the BBA. But, I always worried about what I was doing to the poor fish. Then I saw guaiac boy's idea that the difference between the water in equilibrium with the air and the water in the tank should be about 1.0 ph, and that is about what I had. So, I just gave up on measuring either PH or KH. But, I still wondered. And, I got worried about telling people to jack up the CO2 until the fish objected. Only yesterday did it occur to me that I could easily reduce the CO2 chart to eliminate the KH measurement.

So, I see this as the better way to "measure" CO2 - not perfect, of course, but better.
 

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Chalk me up as another who the charts did not work for. I have been very happy since using the rested tank water method as first suggested to me by guaiac_boy a couple of months ago. I continue to use my ph meter as a guidline to work from.

JR
 
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