Need to understand CO2, KH, PH relationship - The Planted Tank Forum
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post #1 of 22 (permalink) Old 03-04-2019, 11:09 PM Thread Starter
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Exclamation Need to understand CO2, KH, PH relationship

I am not able to understand relationship between CO2, kH and pH. From whatever I have read so far my tank is not behaving the same way.

I have 75g tank with DIY CO2.
Tap water stats -
pH - 7.4
kH - 4dKH
Rhino CO2 color - blue

Tank water stats -
pH - 6.0 (or even lower)
kH - 1dKH (1st drop of API kit is yellow)
Rhino CO2 color - yellow

How is this possible? Rhino says I have lot of CO2 but kH tells me 1dKH. pH has dropped to 6 which tells me there is carbonic acid in the water because of CO2 dissolution.

Any ideas would be helpful.
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post #2 of 22 (permalink) Old 03-04-2019, 11:32 PM
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Don't bother with the charts and that KH, CO2, ph relationship. It really isn't helpful at all for what we do, as there are other factors that throw it off. Instead, most of us just go with the amount of ph drop between the off gassed tank water and peak co2 output.

Take a cup of water from your tank and let it sit for 24-48 hours and then test the ph. In that amount of time all of the co2 will have offgassed from the water. From there a peak 1.0 drop in ph puts you right around 30 ppm. You can push it farther if you want more....most try for between 1.2 -1.5......depending on your livestock. You have to up it slowly though and watch to make sure your fish aren't gasping at the top. If they are, you've gone too far. Back it off a little and leave it there. Test ph on a regular basis as it usually changes a little over time and could leave you with more or less co2.


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post #3 of 22 (permalink) Old 03-05-2019, 01:22 PM
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Quote:
Originally Posted by elusive77 View Post
Don't bother with the charts and that KH, CO2, ph relationship. It really isn't helpful at all for what we do, as there are other factors that throw it off. Instead, most of us just go with the amount of ph drop between the off gassed tank water and peak co2 output.

Take a cup of water from your tank and let it sit for 24-48 hours and then test the ph. In that amount of time all of the co2 will have offgassed from the water. From there a peak 1.0 drop in ph puts you right around 30 ppm. You can push it farther if you want more....most try for between 1.2 -1.5......depending on your livestock. You have to up it slowly though and watch to make sure your fish aren't gasping at the top. If they are, you've gone too far. Back it off a little and leave it there. Test ph on a regular basis as it usually changes a little over time and could leave you with more or less co2.
Beautiful tank shots btw! love your journal!. I really wish this community would stop quoting absolute pH drop ranges. Your max CO2 concentration(can be less) can be loosely estimated measuring the change in pH from degassed Tap versus co2 injected tank water. A rough conversion of pH to CO2 can be estimated based on the original off gassed kH of the water and converting the pH change to a CO2 concentration. That is what Tom Barr's (ph, kh, co2) chart is all about.

An absolute pH drop is meaningless unless we know what the kH of the water was to begin with. So saying shoot for a 1ph or 2ph drop and ignoring the charts is not very helpful.

1) Off gas tap water in open container for 24 hours and then measure its pH.
2) Measure your in tank pH and take the difference
3) Measure your kH of off gassed Tap water using the API test kit or other.
4) Lookup your CO2 levels on the (kh, pH , CO2) tables (google this chart from Tom Barr).

This method will give you a reasonable estimate of your max CO2 levels in your tank. The concentration of CO2 may be a little lower as there could be other sources of hardness in your source water that is not Carbonates but we basically assume those are not high.


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post #4 of 22 (permalink) Old 03-05-2019, 02:09 PM
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Quote:
Originally Posted by cl3537 View Post
Beautiful tank shots btw! love your journal!. I really wish this community would stop quoting absolute pH drop ranges. Your max CO2 concentration(can be less) can be loosely estimated measuring the change in pH from degassed Tap versus co2 injected tank water. A rough conversion of pH to CO2 can be estimated based on the original off gassed kH of the water and converting the pH change to a CO2 concentration. That is what Tom Barr's (ph, kh, co2) chart is all about.

An absolute pH drop is meaningless unless we know what the kH of the water was to begin with. So saying shoot for a 1ph or 2ph drop and ignoring the charts is not very helpful.

1) Off gas tap water in open container for 24 hours and then measure its pH.
2) Measure your in tank pH and take the difference
3) Measure your kH of off gassed Tap water using the API test kit or other.
4) Lookup your CO2 levels on the (kh, pH , CO2) tables (google this chart from Tom Barr).

This method will give you a reasonable estimate of your max CO2 levels in your tank. The concentration of CO2 may be a little lower as there could be other sources of hardness in your source water that is not Carbonates but we basically assume those are not high.
The reason I and most others here suggest absolute ph drop is because it is a simple and accurate way to get to the right co2 level. There are less unknown variables to worry about. Yes, the chart is useful to get an understanding of the relationship between ph, kh, and co2. But it's very hard to be accurate using the chart as OP is seeing.

If I use your method above on my tank for example, I show a max co2 concentration of 190ppm. My fish would not be alive if that were the true amount of co2. As you said there are other things in water that affect this. You can assume they are not high but for most people, especially those of us who use tap water, they actually are.

Believe me, I tried for a long time when I was starting out to use the chart, based on stuff I read from Tom. And somehow my co2 was always low. I could never seem to get it right.....until someone told me to ditch the chart and shoot for a 1 pt drop in ph. It made a huge difference in my tank. So I'm just trying to pay that forward here.


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post #5 of 22 (permalink) Old 03-05-2019, 02:19 PM
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I recognize that we do need to have some guidelines as we start but from there, I feel we are often too involved with meeting standards which do not take into account how much things like water change the answers. We often read that PH is critical but I find it to be one that I don't need to worry as much as I read. I find PH only important as a part of the water parameters that give me some info but not as a critical item to set. I find hardness and buffering are far more important and those change how my fish act far more. I feel they are the two which bother my fish as they control how the swim bladder works to regulate liquids in their body.
I do start out with a simple set of values that I might "like" to get but then my water and my fish/plants are far more important than the "standard" answers that I read. I might read that "X" of this and "Y" of that is great water but what is the value in that if my water is not what's recommended? I have to live with the water I have, not what the writer's on each coast may have. My other choice is too go into a fulltime fight with nature to change my water!
My plan is to make this a simple, fun project and that means be happy with the work I do, NOT work myself to death trying to do what somebody else finds right! Step one is to find what I have and then fit plants and fish into that rather than go for some "ideal" which has nothing to do with what I have or want.
I have been raising and breeding African cichlids which are great in hard alkaline water but I wanted some plants to make the fish look better and to challenge myself a bit more. So I found the plants which are more likely to thrive in my water and tried it. With CO2 added, I found my fish could still do well when the PH was pressed down from 7.8 to about 6.8. They DO NOT have to have high PH as I often read! I add CO2 gradually as I watch my fish and adjust as needed.
I choose to do what works in MY water and ignore what might work in the water in other places. I allow myself the freedom to have fun!
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post #6 of 22 (permalink) Old 03-05-2019, 02:37 PM
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Quote:
Originally Posted by elusive77 View Post
The reason I and most others here suggest absolute ph drop is because it is a simple and accurate way to get to the right co2 level. There are less unknown variables to worry about. Yes, the chart is useful to get an understanding of the relationship between ph, kh, and co2. But it's very hard to be accurate using the chart as OP is seeing.
Edit: You are right, I won't be referencing the chart anymore it only works if the assumptions hold, no other acids, bases or buffers.
1 pH drop barely works accurately as well, but at least if you use your tank degassed pH(not your source degassed) and drop it 1 pH in moderate kH and 0.7 in high kH you can get to a decent starting point and then adjust by plant and fish health.

ph Error, kH error, differences in CO2 atmospheric pressure, acids and based and buffers in the tank, there are so many errors, getting absolute CO2 ppm values is just not possible with common hobbyist equipment.


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Last edited by cl3537; 03-06-2019 at 04:40 AM. Reason: Got Informed!
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post #7 of 22 (permalink) Old 03-05-2019, 02:56 PM
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Originally Posted by PlantedRich View Post
"I do start out with a simple set of values that I might "like" to get but then my water and my fish/plants are far more important than the "standard" answers that I read. ..... I found my fish could still do well when the PH was pressed down from 7.8 to about 6.8. They DO NOT have to have high PH as I often read! I add CO2 gradually as I watch my fish and adjust as needed.
I choose to do what works in MY water and ignore what might work in the water in other places. I allow myself the freedom to have fun"
+1 to this approach. But first you need to know what is in your water and then select your aquarium fish and plants based on your water.

Lots of hard water people use RODI but that is a whole new level of effort required.


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post #8 of 22 (permalink) Old 03-05-2019, 03:18 PM
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The charts are something that I check each time I move but then when I find the answers to be totally wrong, I stop thinking about the charts! I've long forgotten what the charts and testing showed as my CO2 content but it was just too far off to mean anything, something like my fish were living in 250 PPM or something that I knew was not true.
What we need to know when we read anything is who and where that info is developed. It is not that this info is not correct for the situation that it meets but when it becomes obvious that those conditions/situations are not what we have, we need to stay open to looking for different information.
We do need to know about the world situation but when dealing with our tank, we need to know and deal with it as a local situation.
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post #9 of 22 (permalink) Old 03-05-2019, 03:39 PM
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Lots of hard water people use RODI but that is a whole new level of effort required.
Not at all true - the use of remineralized R/O removes so many variables and really simplifies tank maintanence. It's much less effort than trying to manage with all the things that could be in tap water.



While the chart is an ok guide, it doesn't take into account other variables that affect the PH (like tannins and other acids). As long as the KH isn't out of whack (which, unfortunately it is in this case), the 1 PH drop method is really the most accurate way to gauge the CO2 level.

Almost everyone in this thread would benefit from reading this article: https://www.advancedplantedtank.com/...co2-level.html


Also, @sagarjoshi - it's probably best to stick to one thread when trying to handle tank problems.
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Last edited by Quicksilver2299; 03-05-2019 at 04:02 PM. Reason: Added content
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post #10 of 22 (permalink) Old 03-05-2019, 04:39 PM Thread Starter
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Thanks everyone!

So if I look at the chart, given my kH I need make sure I inject CO2 such that my pH remains between 6.5-6.9.

Now, what about water changes? Itís a 75g tank. Wonít water changes increase the pH? How small should the water changes be if I want to see minimal fluctuations?

Or should I not dose CO2 a day before water changes so that pH rises, eliminating fluctuations during water changes?


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post #11 of 22 (permalink) Old 03-05-2019, 04:57 PM
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Originally Posted by sagarjoshi View Post
So if I look at the chart, given my kH I need make sure I inject CO2 such that my pH remains between 6.5-6.9.

Now, what about water changes? It’s a 75g tank. Won’t water changes increase the pH? How small should the water changes be if I want to see minimal fluctuations?

Or should I not dose CO2 a day before water changes so that pH rises, eliminating fluctuations during water changes?

Even with your KH, I'd ignore the chart. Something in your tank is lowering the KH, likely meaning that a stronger acid than CO2 is present.

Without diving into too much detail, I wouldn't worry about PH fluctuations during a water change. The PH will equalize with the KH, aside from the effect of the CO2. Focus more on KH fluctuations, as they are a much bigger concern for plants and livestock. Keep the CO2 as constant as possible - no need to stop for a water change as it would do more harm than good.

Here's another article about water parameters from the site I referenced earlier: https://www.advancedplantedtank.com/ph-kh-gh-tds.html

I'd recommend bookmarking that site and using every article in it as a reference. The info in it has helped dramatically increase my success.
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post #12 of 22 (permalink) Old 03-05-2019, 05:46 PM
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iirc, that chart assumes only carbonates (?) contribute to kh. so it may actually underestimate co2 concentration. my tap water kh is pretty steady with a kh of 6 and 7 gh, pH 7.6 - 7.8. i use the >1.0 pH drop and plant color and growth have never been better.
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post #13 of 22 (permalink) Old 03-06-2019, 04:30 AM
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Originally Posted by Quicksilver2299 View Post
Not at all true - the use of remineralized R/O removes so many variables and really simplifies tank maintanence. It's much less effort than trying to manage with all the things that could be in tap water.
Agreed, it removes uncertainty in water parameters but it still requires significant time, expense, and maintenance. Are you using one?
Quote:
While the chart is an ok guide, it doesn't take into account other variables that affect the PH (like tannins and other acids). As long as the KH isn't out of whack (which, unfortunately it is in this case), the 1 PH drop method is really the most accurate way to gauge the CO2 level.
The chart really fails in so many situations including mine as I have recently discovered, I won't be referencing it anymore. Tom Barr said it will often overestimate CO2 concentration(especially at high kH) but he hasn't seen a situation where it underestimated it. Well when your tap pH is higher than calculated from kH because of other sources of basicity(buffer or otherwise) it can understimate your CO2 as it doesn't even account for the ph change only the final pH.

Formula used in the chart 3*kh*10^(7-ph)=[Co2]ppm

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Quote:
Originally Posted by milesm View Post
iirc, that chart assumes only carbonates (?) contribute to kh. so it may actually underestimate co2 concentration. my tap water kh is pretty steady with a kh of 6 and 7 gh, pH 7.6 - 7.8. i use the >1.0 pH drop and plant color and growth have never been better.
At 7.8ph and kh=6 your water at the high end would actually work with the chart ~30ppm with ph drop down to ~6.8


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Last edited by cl3537; 03-06-2019 at 04:39 AM. Reason: Typos
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post #14 of 22 (permalink) Old 03-06-2019, 11:59 AM
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Originally Posted by elusive77 View Post
Believe me, I tried for a long time when I was starting out to use the chart, based on stuff I read from Tom. And somehow my co2 was always low. I could never seem to get it right.....until someone told me to ditch the chart and shoot for a 1 pt drop in ph. It made a huge difference in my tank. So I'm just trying to pay that forward here.
You've said a mouthful there.

And many would benefit from reading that very carefully and taking it to heart.

For much of what we do, the absolute value is meaningless. What we do is part science, and part art.

IMO, the key to success is fine tuning relative values, and then, most importantly, listening to what the tank is telling you. And this goes for both CO2 and ferts too.

Throw out preconceived notions, and watch the plants. Then it's trial and error to dial in the best balance for YOUR tank.
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post #15 of 22 (permalink) Old 03-06-2019, 12:29 PM
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You've said a mouthful there.

And many would benefit from reading that very carefully and taking it to heart.

For much of what we do, the absolute value is meaningless. What we do is part science, and part art.

IMO, the key to success is fine tuning relative values, and then, most importantly, listening to what the tank is telling you. And this goes for both CO2 and ferts too.

Throw out preconceived notions, and watch the plants. Then it's trial and error to dial in the best balance for YOUR tank.
Ita a bitter pill to swallow that we have to get in the ballpark and then trial and error just about everything due to accuracy issues or go to RODI system and know and control everything.
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