The Planted Tank Forum banner

1 - 20 of 25 Posts

·
Registered
Joined
·
527 Posts
Discussion Starter · #1 ·
I thought I understood this chart but one point of confusion to me is that the PPM numbers for "too little CO2" in yellow can be very high, while the PPM numbers for "too much CO2" are very low. Is this chart accurate? Is this just how the chemistry works?

I have a Seiryu rock in my tank which appears will cause some KH drift throughout the week so I am trying to see how I can use this chart to plan. But not sure how this chart is used. What numbers do you plug in? Your after CO2 pH and KH or your nominal pH and KH?

download.jpg
 

·
Registered
Joined
·
375 Posts
My understanding is that the ppm numbers are the amount of CO2 required to drop the pH to a given level. KH resists changes in pH, so you need more CO2 to drop the pH to the same value when you have high KH. In contrast it takes very little CO2 to drop the pH in low-to-zero KH water (hence why RO water often measures quite acidic as it has absorbed a small amount of CO2 from the air).

At any KH, the optimum CO2 level is about 15-30 ppm, according to the chart. The idea is that 30ppm of CO2 will drop the pH all the way down to 6.3 if you have 2KH, but will only drop to ph7.2 if you have very hard (high KH) water.

But wait, now I'm confused! Are the yellow and red sections not the wrong way round in that chart???? How can 2ppm CO2 be "too much CO2" and 223ppm be "not enough CO2"???

Ok, give up. Let's wait for an expert to come along....
 

·
Registered
Joined
·
527 Posts
Discussion Starter · #5 ·
Does KH "absorb CO2"? Why wouldn't your PPM of CO2 simply be determined by your bubble count/flow rate? That's the whole confusing part of this. Since it says you have to "drop pH by 1 to get 15-30ppm".
 

·
Super Moderator
Joined
·
11,986 Posts
I thought I understood this chart but one point of confusion to me is that the PPM numbers for "too little CO2" in yellow can be very high, while the PPM numbers for "too much CO2" are very low. Is this chart accurate? Is this just how the chemistry works?

I have a Seiryu rock in my tank which appears will cause some KH drift throughout the week so I am trying to see how I can use this chart to plan. But not sure how this chart is used. What numbers do you plug in? Your after CO2 pH and KH or your nominal pH and KH?

View attachment 903045
This chart is incorrect; the red section should be labeled "too little CO2" while the yellow section should be labeled "too much CO2"

Does KH "absorb CO2"? Why wouldn't your PPM of CO2 simply be determined by your bubble count/flow rate? That's the whole confusing part of this. Since it says you have to "drop pH by 1 to get 15-30ppm".
Carbonate hardness serves as a buffering agent against changes in pH. Essentially, with a higher KH, you would have to inject more CO2 to achieve the same decrease in pH, meaning you'd be injecting more (and have a higher concentration).


The concentration of CO2 is determined by your injection rate, but you also have to take into account a multitude of other factors, such as off-gas, absorption, dissolution rate, other effects on pH such as humid acids and other dissolved organic compounds, etc, so in the end, we just simplify it to reduce the complexity.
 

·
Registered
Joined
·
527 Posts
Discussion Starter · #7 · (Edited)
This chart is incorrect; the red section should be labeled "too little CO2" while the yellow section should be labeled "too much CO2"
Carbonate hardness serves as a buffering agent against changes in pH. Essentially, with a higher KH, you would have to inject more CO2 to achieve the same decrease in pH, meaning you'd be injecting more (and have a higher concentration).
I can't understand how that works. So animal life can handle a higher CO2 PPM because of a high KH? I have not seen that claim directly made.

That's why I was asking if KH "absorbs CO2" and thus you have to put more CO2 in to have 30PPM free dissolved CO2 with a higher KH. But from reading, only a small fraction of a percent turns into carbonic acid.

Given that the chart has this flagrant error of saying too much is too little and too little is too much, I would say that is enough to distrust it until proven otherwise.

My current hypothesis, which I could be wrong as I am still quite ignorant on this issue, is that your concentration of CO2 is simply determined by your flow rate/bubble count minus your off-gassing, regardless of any water chemistry. That is may be altered so slightly as to not matter by KH due to KH absorbing some of the .3% of CO2 converted to carbonic acid. That would need to reach an equilibrium and so perhaps a higher KH would cause a little more CO2 to be converted to acid vs free CO2 dissolved in the water.

Maybe a chemist is on board and can clarify this.
 

·
Registered
Joined
·
5,788 Posts
Throw out the chart and you don't need a chemist.

To get an absolute measure of CO2 it takes expensive equipment. So we are left to our devices.

Which is basically controlling flow of CO2 to reach a relative pH drop. Absolute value really means little to us. And that goes for most everything in the hobby. It's relative values that count.

The trick is dialing in the right pH drop that brings out optimal plant healthy while not harming fish/fauna.

The "standard" folks talk about is a 1.0 pH drop. But optimal can be more or less depending on the tank.

A low light tank with slow growers needs less, a tank driven hard by light with lots of stems needs more.

In my tank (high light), my pH drop is 1.45. And you will find many other "high tech" tanks in the 1.2 to 1.5 range. The charts would tell you my CO2 concentration is 100+, and all the fauna would be dead, but in practice that is not the case.

So don't get too bogged down with the science. I've seen brilliant science minds that can't grow plants. Make choices based on what you see, not what you think or calculate should happen.
 

·
Registered
Joined
·
527 Posts
Discussion Starter · #9 ·
I think you are correct. I am going to just focus on a stable bubble rate and the drop checker being green, adjusting if it becomes bluish or yellowish (trying not to err on that side). I think that's what the drop checker checks, green = -1pH I think.
 

·
Super Moderator
Joined
·
11,986 Posts
I can't understand how that works. So animal life can handle a higher CO2 PPM because of a high KH? I have not seen that claim directly made.

That's why I was asking if KH "absorbs CO2" and thus you have to put more CO2 in to have 30PPM free dissolved CO2 with a higher KH. But from reading, only a small fraction of a percent turns into carbonic acid.
Higher KH serves to buffer against pH changes. Injecting CO2 will affect pH, but the amount of pH drop will be determined by how much buffering capability your water has.



If your water has a high amount of buffering capability (high kH), then by the time you inject enough CO2 to achieve (say) a pH of 6.5, you would have much higher amounts of CO2 than if your kH was lower.



My current hypothesis, which I could be wrong as I am still quite ignorant on this issue, is that your concentration of CO2 is simply determined by your flow rate/bubble count minus your off-gassing, regardless of any water chemistry. That is may be altered so slightly as to not matter by KH due to KH absorbing some of the .3% of CO2 converted to carbonic acid. That would need to reach an equilibrium and so perhaps a higher KH would cause a little more CO2 to be converted to acid vs free CO2 dissolved in the water.

Maybe a chemist is on board and can clarify this.
Water chemistry will play a role, as indicated above, and is not solely dependent on flow rate/dissolution rate/off gassing rate. The only way it would not play a role is if you were trying to inject CO2 into water with no buffering capability (e.g. distilled water)
 

·
Registered
Joined
·
527 Posts
Discussion Starter · #11 ·
If your water has a high amount of buffering capability (high kH), then by the time you inject enough CO2 to achieve (say) a pH of 6.5, you would have much higher amounts of CO2 than if your kH was lower.
I understand the buffering relationship.

What I am asking is if the "extra amount" of CO2 you refer to as "you would have much higher amounts of CO2 than if your kH was lower" represents a higher concentration of CO2 than if your starting pH is lower. And why this would not be a higher risk to animal life than lower concentrations of CO2, regardless of water pH.

For example:

tank___starting-pH___CO2 required to lower -1pH (ppm)
A_________7________x
B_________8________y

and assuming the difference of pH in A and B is due to KH, its the relation that y > x

Relative to tank A, for tank B with starting pH of 8., where does the "excess" (y-x) CO2 that's needed to drop -1pH to 7 go? Does that become more dissolved CO2 in the tank water? This would imply that tank B runs at a higher concentration (PPM) of CO2 to meet the pH goal and so why is that just as safe as tank A?

Or does all the "excess" (y-x) CO2 become carbonic acid and react (or some other chemical change) with bi/carbonates (KH) and get "locked out" from being dissolved CO2, so that dissolved CO2 concentrations at -1pH are the same for both tank A and tank B?
 

·
Super Moderator
Joined
·
11,986 Posts
I understand the buffering relationship.

What I am asking is if the "extra amount" of CO2 you refer to as "you would have much higher amounts of CO2 than if your kH was lower" represents a higher concentration of CO2 than if your starting pH is lower. And why this would not be a higher risk to animal life than lower concentrations of CO2, regardless of water pH.
So, there are several factors to consider.


Firstly, dissolution of CO2 into water creates carbonic acid, but this is relatively short lived, and the equilibrium favours bicarbonate formation, as you have already mentioned and are aware.



Secondly, the kH/pH/CO2 relationship only holds true if (bi)carbonate anions are the only contributing species to carbonate hardness (which is not the case in an aquarium).


However, if this is indeed the case, then the chart is quite simple. If you measure aquarium A to have a pH of 7, while aquarium B to have a pH of 8, and given that their kH are the same, then aquarium A will have more CO2 than aquarium B. You are essentially forcing the equilibrium to the right, which favours proton/bicarbonate formation.


Now, consider the case where aquarium A has a kH of (say) 2, and aquarium B has a kH of (say) 5, and we measure them both to have a pH of 7. Again, this is easy to figure out, because the chart would indicate aquarium A to have a CO2 concentration of 6 ppm, while aquarium B to have 16 ppm of CO2.



But why does aquarium B have higher CO2? This is in part due to the fact of how carbonate hardness is derived. The presence of bicarbonate anion is usually from carbonates (e.g. typically calcium carbonate in aquatic systems). The calcium carbonate contributes carbonate (eventually bicarbonate), but does not generate any protons (e.g. does not create a more acidic condition). However, that being said, the pH of carbonate solutions also tends towards alkaline.


In any case though, for aquarium B now, with a higher kH, in order to achieve a pH of 7, one would need to inject more CO2, which would force more carbonic acid (and in turn, bicarbonate and protons) to be formed. However, this equilibrium is unfavourable, due to the presence of pre-existing bicarbonate, which is why you would require more CO2 to achieve the same pH.


As a direct result of injecting more CO2, you will also have more free CO2 in the water (prior to it forming carbonic acid); it is this (free CO2) that is dangerous to livestock.



In summary; with higher kH, to affect the same decrease in pH, you would need to inject more CO2, which ultimately leads to higher CO2 concentration in the water, which can be lethal to livestock.


For example:

tank___starting-pH___CO2 required to lower -1pH (ppm)
A_________7________x
B_________8________y

and assuming the difference of pH in A and B is due to KH, its the relation that y > x

Relative to tank A, for tank B with starting pH of 8., where does the "excess" (y-x) CO2 that's needed to drop -1pH to 7 go? Does that become more dissolved CO2 in the tank water? This would imply that tank B runs at a higher concentration (PPM) of CO2 to meet the pH goal and so why is that just as safe as tank A?

Or does all the "excess" (y-x) CO2 become carbonic acid and react (or some other chemical change) with bi/carbonates (KH) and get "locked out" from being dissolved CO2, so that dissolved CO2 concentrations at -1pH are the same for both tank A and tank B?
In this scenario, if we assume that the kH is the same for both aquariums, then y is actually less than x. You can see this from the chart (e.g. given a kH of 2, a pH drop from 7.5 to 7.0 requires an increase of 4 ppm of CO2, but from 7.0 to 6.5, requires 14 ppm of CO2). However, as the pH scale is logarithmic, this makes sense. A pH drop of 0.5 would be 10^0.5 = 3.16 times more CO2, (2 pm * 3.16 = roughly 6 ppm, and again, 20 ppm is about 3.16 times of 6 ppm). The increase in effective CO2 concentration (e.g. 6 ppm vs 20 ppm) is because you are trying to push the equilibrium to favour protonation. This equilibrium is also why you cannot achieve very low pHs with CO2 injection (the equilibrium, at a certain point, will favour to the left, which results in CO2 off gassing).


Hopefully, this all makes sense; as already mentioned, this hobby is about fishkeeping and plant care, not trying to understand all the chemistry behind it :D


However, if that is your inclination, then by all means!
 

·
Registered
Joined
·
527 Posts
Discussion Starter · #13 ·
In this scenario, if we assume that the kH is the same for both aquariums, then y is actually less than x. You can see this from the chart (e.g. given a kH of 2, a pH drop from 7.5 to 7.0 requires an increase of 4 ppm of CO2, but from 7.0 to 6.5, requires 14 ppm of CO2). However, as the pH scale is logarithmic, this makes sense. A pH drop of 0.5 would be 10^0.5 = 3.16 times more CO2, (2 pm * 3.16 = roughly 6 ppm, and again, 20 ppm is about 3.16 times of 6 ppm). The increase in effective CO2 concentration (e.g. 6 ppm vs 20 ppm) is because you are trying to push the equilibrium to favour protonation. This equilibrium is also why you cannot achieve very low pHs with CO2 injection (the equilibrium, at a certain point, will favour to the left, which results in CO2 off gassing).


Hopefully, this all makes sense; as already mentioned, this hobby is about fishkeeping and plant care, not trying to understand all the chemistry behind it :D


However, if that is your inclination, then by all means!
I appreciate all this! I very much enjoy fishkeeping without the chemistry but in this case I want to know more as my fish's lives depend on it. I feel the need to have some basis to make decisions about levels of CO2.

I guess the thing I still don't get is the supposed "magic of -1pH" that the 2HR Aquarist and others promote. To me that is a very BROAD rule of thumb and in fact most people may not want to target exactly the "magical -1pH". If my KH is high it sounds like I may not want to drop -1pH, I might want to drop -.5pH for example, to get to my target CO2 PPM. That's why I was trying to use the chart and started this thread, was to try to figure this out. Of course one of the base problems is I chose an incorrectly labeled chart to start off with ... oops.

The other thing that is confusing is that people say, as you did in your response, "if your KH is x and your pH is y, then your CO2 is z". That's only the case if you've added CO2! If that is your water parameters out of tank/tap with no CO2 augmentation, then there is only 2-3PPM CO2. I think when people say that (including your comments above), they do mean AFTER CO2 has been added. The 2HR Aquarist has an article on that that very thing posted on this thread (subject of "wrong way to read the drop checker chart").

Thanks for the replied, its starting to sink in!
 

·
Premium Member
Rank Amateur
Joined
·
1,370 Posts
I appreciate all this! I very much enjoy fishkeeping without the chemistry but in this case I want to know more as my fish's lives depend on it. I feel the need to have some basis to make decisions about levels of CO2.

I guess the thing I still don't get is the supposed "magic of -1pH" that the 2HR Aquarist and others promote. To me that is a very BROAD rule of thumb and in fact most people may not want to target exactly the "magical -1pH". If my KH is high it sounds like I may not want to drop -1pH, I might want to drop -.5pH for example, to get to my target CO2 PPM. That's why I was trying to use the chart and started this thread, was to try to figure this out. Of course one of the base problems is I chose an incorrectly labeled chart to start off with ... oops.

The other thing that is confusing is that people say, as you did in your response, "if your KH is x and your pH is y, then your CO2 is z". That's only the case if you've added CO2! If that is your water parameters out of tank/tap with no CO2 augmentation, then there is only 2-3PPM CO2. I think when people say that (including your comments above), they do mean AFTER CO2 has been added. The 2HR Aquarist has an article on that that very thing posted on this thread (subject of "wrong way to read the drop checker chart").

Thanks for the replied, its starting to sink in!
Hi! I had made this long reply on your other thread when you asked a similar question about "buffering" until I realized my reply didn't really address it and thus, was scrapped.. and I had the same problem sort of understanding it altogether until reading some of yours and @Darkblade's posts. Now I had to rewrite it as it seemed more apt for this thread. You see, my original reply revolved around the chart itself and it might just address your questions better here as well. Let's see if I can remember most of what I wrote and along the way, clear up my own interpretations of it... and anyone else reading, please correct me if I'm wrong, I will gladly edit this post with credit.

First of all, let's clarify the most blatant error on the earlier chart with one from a better source:



and the accompanying thread: https://barrreport.com/threads/co2-ph-kh-table.10717/

and the first post by Tom:
"Measured your KH, then see how much you need to reduce the pH to get your target CO2 ppm.

Say you tap water is a KH of 5m say you want 35-40ppm of CO2, you should add enough to get the pH to 6.6 and be able to keep it there.

Warning, KH may not be entirely carbonate hardness. This means you will think and believe you have MORE than you actually do, thus you may be under dosing CO2.
This issues will never be the reverse, eg, you are adding more CO2 than you think.

So the error is always on the safe side usign this method.

As the KH in your tap drops, say your KH is 1-2 degrees, there's just not much room for other sources of KH other than carbonate, at 4-5 and above, there may be.
So assuming most of the KH is carbonate hardness for a KH or 1 degree is likely okay.
"
As you so importantly pointed out, the intention is to pull numbers AFTER co2 injection!! Now, I read his quote to mean if your KH is 5 and you add CO2 to get your ph to 6.6, you "have enough". Then, he turns around and says if the KH is other than carbonate, you will 'think' that you have more than you do. What's not said here is if .. you think this way, your starting ph MUST have been 7.6 .. because if you follow the chart, 7.6 is where there is properly only ~3ppm of CO2 as you yourself has noted is how you're supposed to approach the chart BEFORE injection!

[I must admit for this part, I'm a little confused by what he means about 'carbonate'. What are the other sources of KH? Someone feel free to help me out here.]

Keep in mind, this scenario works IF only you had water that somehow isolated from that "other stuff." Oh wait.. you do! I think this is how drop checkers are supposed to work. Isolate 4dKH of water made from distilled water and baking soda -- from the excellent
tank___starting-pH___CO2 required to lower -1pH (ppm)
A_________7________x
B_________8________y

and assuming the difference of pH in A and B is due to KH, its the relation that y > x
[/quote]

Wait, what are you doing here. You're not guaranteeing your starting ph is at the right point of the chart where your CO2 is at 3ppm. OR, if you are, you're not considering your ending-pH. Because if you did, why would y > x?

or similarly, this part:
I guess the thing I still don't get is the supposed "magic of -1pH" that the 2HR Aquarist and others promote. To me that is a very BROAD rule of thumb and in fact most people may not want to target exactly the "magical -1pH". If my KH is high it sounds like I may not want to drop -1pH, I might want to drop -.5pH for example, to get to my target CO2 PPM. That's why I was trying to use the chart and started this thread, was to try to figure this out.
I think the confusion arises because, against what you realized, you're attempting to read the chart PRE-injection. So, if your KH is high already pre-injection, you need to alter your start point to where it says 2-3ppm co2 and start there and get your -1pH. So yea, solely based on the chart, you still want your -1ph in high KH. Of course that statement goes out the window too if you have "other things" lowering your ph. See why looking at the chart in absolutes is pretty flawed?

---

Additionally, the -1ph idea AND the chart works because it's different ways to look at a formula possibly close to this one that I dug up from this post waaay back in time..

[url]http://fins.actwin.com/aquatic-plants/month.9707/msg00211.html
CO2 = 12.839 * dKH * 10^(6.37 - pH)

run the numbers.. I'll wait :) Every slot on the chart roughly corresponds to that. As I don't know the EXACT formula Tom used to make the chart, I suppose the numbers don't match perfectly.

But what does the formula also force us to do? It makes us realize the numbers are ONLY for pH in relation to what the CO2 is doing. Given a known CO2 starting point (equilibrium), a known KH, you get a certain pH. The formula doesn't have a parameter of pH from something else not caused by CO2! It locks us back to the 3ppm starting point and the relevant ph starting point.

https://www.aquariumadvice.com/foru...o2-really-cause-ph-swings-edit2-no-52085.html
This thread goes into more discussions along with a similar - even more simplified formula in post #4, but regardless of how people say it, it's important to interpret the chart as simply one really inaccurate way to sort of get you in the "ballpark" of the right co2 in your tank. As Dennis and most of the old hands say, watch your plants, and definitely watch your fish as that is your priority. Wrong CO2 shows up pretty fast in fish. Wrong CO2 in plants, in time, you will start to notice the nuances -- just don't confuse them for dosing issues! :grin2:
 

·
Registered
Joined
·
2,402 Posts
Excellent posts, so far.

Although most don't approach it this way, there is another way of looking at this KH conundrum that might increase understanding (some folks learn from different angles). It is taking into account the concise description in the "2Hr Aquarist" link in my post #4, above.

I target a pH of 6.5 (for several reasons) and, secondarily, move my CO2 to ~30ppm by way of a 1-point pH drop to the 6.5. However, as others have said or implied, this is a relative measurement in view of the absolute KH (which is comprised of more than bicarbonate). So, I de-gas a container of tank water and then measure the pH and dKH of that de-gassed water. I determine how much bicarbonate (if any) is necessary (via the dKH reading) to add to reach 7.5 in that de-gassed water. I then add the appropriate amount of bicarbonate (the dKH needed to reach the 7.5 pH in the de-gassed water) to my tank, and adjust CO2 until I reach pH 6.5 ...the golden 1-point drop.

Once I have this CO2 injection rate set, the KH drift doesn't matter because the CO2 rate initially set, above, is the CO2 rate that delivers ~30ppm no matter what KH and pH do. Of course, KH drifts , so I have to adjust KH to maintain my 6.5 pH (during water changes), but this doesn't affect my CO2 level.

I do want to add one caveat to all of this: Ph and dKh readings are highly dependent upon the accuracy of our test kits. pH meters are best and there are ways to increase dKH precision of our basic KH test kits.
 

·
Registered
Joined
·
2,402 Posts
To target pH 6.5.

The explanation was mainly to show how to include the KH reading at both ends of the CO2 target for bracketing purposes. Just another angle to view an alternative to the inherent potential KH problems in the chart.
 
1 - 20 of 25 Posts
Top