# CO2, pH, and dKH question



## dukydaf (Dec 27, 2004)

Hello and welcome to the forum and planted tanks. Things might be confusing at first and you may encounter contradictory opinions. I would like to suggest the articles from The Skeptical Aquarist as a good overview (I do not agree with all his ideas). There you have many articles about water parameters.

Now answering your questions.

*That chart only works in aquariums that already have CO2 injected*. This is because it assumes that the majority of the acid is carbonic acid and thus a lower pH is because of disolved CO2. A starting pH 7.3, you should not worry about the pH in the aquarium, it is good for most plants and fish. Pure water has a pH of 7. KH is a measure of alkalinity, which is to say a buffer that prevents the pH from being lowered by acids. 

If you do not add CO2 and provide good water surface movement, your aquarium concentration will have ~the concentration of atmospheric CO2
*If you add CO2, your dKH will stay the same only your pH will be lower.* KH can change but not from CO2 and for the most part it does not widely vary.

Hope you find a helpful and friendly community here, as I have. 

Regards


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## sohankpatel (Jul 10, 2015)

I advise against a PH controller, i think that they are overrated, go with a timer and get a drop checker. The drop checker has fluid that you put in it. It is blue when co2 is below 20-30PPM co2, it is yellow when there is more than that. Lime green means that you have hit the sweet spot, if you are injecting co2, a drop checker is one of the most important things you can buy.


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## Diana (Jan 14, 2010)

Water companies often add other chemicals to maintain a pH in the alkaline range. Low pH is bad for the pipes. 
When this is happening you cannot use the KH-pH chart to figure out the CO2 level. As noted above, that chart works when the only material controlling the pH is carbonate/CO2 and the related chemicals. 

What is the GH of the water? This is more important than the KH or pH for the fish. 

For soft water fish the GH and KH should usually be around 5 degrees or less, as low as 1-2 degrees for some of the most particular of fish. 
If the fish came from a black water river, then peat moss, oak leaves, alder cones, Indian Almond leaves, wood (many kinds) will add the organic acids these fish need. 
Once these needs are met, the pH will highly likely be in the right range for the fish. Adding CO2 will drop the pH a bit lower, too. 

To get the right CO2 level, you can test the pH once the tank is stabilized, then add CO2 and test the pH again, after the CO2 is well circulated. A difference of 1 unit in the pH = 30 ppm of CO2. 
When you add fish to this tank they will not be used to CO2. Either acclimate them to CO2 while they are in a quarantine tank, or else stop the CO2 and gradually increase it so the fish can get used to it. Better to do this in a quarantine tank. If you stop and start the CO2 in the main tank, this is one thing that encourages algae.


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## Audionut (Apr 24, 2015)

dukydaf said:


> *That chart only works in aquariums that already have CO2 injected*. This is because it assumes that the majority of the acid is carbonic acid and thus a lower pH is because of disolved CO2.


The chart works on the chemical equilibrium between CO2 dissolved in water and Karbonate hardness. It will provide the same level of accuracy with CO2 injected into the tank, as it will without CO2 injected into the tank. 

The inaccuracy arises where people measure total alkalinity and assume this means the same as carbonate hardness. If we consider that the CO2 level at equilibrium is 3ppm, then using the chart we can consider the KH to be closer to 2 dKH for 7.3 pH.


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## dukydaf (Dec 27, 2004)

Audionut said:


> The chart works on the chemical equilibrium between CO2 dissolved in water and Karbonate hardness. It will provide the same level of accuracy with CO2 injected into the tank, as it will without CO2 injected into the tank.


Thank you @Audionut for the your comment. Indeed, if you have non carbonate KH you will overestimate how much CO2 you have in the aquarium. The chart also links CO2 to pH.

I still see no point in using this chart in non CO2 fertilized aquariums. Assume you do not have enough CO2. For example say one person starts with a tap water of 7.5 and KH of 9, which gives ~ CO2 8.5 ppm.

Scenario I
However this person does not change the water on a regular basis and tries to create a blackwater setup with all kinds of leaf litter, driftwood, peat and tannins. All these acids accumulate and manage to lower the pH to 6.4. The KH remains the same, 9, so according to the chart this person now has ~CO2 107ppm. Poor fish must be gasping plants growing like crazy, though no CO2 was added. Nothing like this, in this setup the %carbonic acid which lowered the pH is minimal, the chart is totally inaccurate and useless.

Scenario II
Assume the KH=9 is the actual carbonate hardness. This person starts to add CO2 and cranks it too high. Now only dissolved CO2 is responsible for the change in pH. The pH drops to the same 6.4. Otos jump out of the aquarium and the Betta is happy that it belongs to Osphronemidae family.

I would say in our aquariums with a moderate KH and weekly water changes the difference in CO2 levels given by non-carbonate KH is less than 10ppm. Hope this clarified what I tried to say in my first post.


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## Audionut (Apr 24, 2015)

dukydaf said:


> I still see no point in using this chart in non CO2 fertilized aquariums.


Agreed, to an extent. Because CO2 will always try and maintain equilibrium with the surrounding atmosphere. Henry's Law. So determining the CO2 levels with the chart is pointless, since CO2 levels will be at equilibrium with the atmosphere (or thereabouts).

However, CO2 - KH - pH have a relationship. If you know the value of (any) two, you can determine the third. So as above, we assume (non CO2 injection) that the equilibrium of CO2 with the atmosphere is 3ppm, and the pH is 7.3, then we can determine that the actual Karbonate hardness is around 2 dKH. Most people only try and determine CO2 with pH and KH, but you can determine pH with CO2 and KH, or KH with CO2 and pH.




dukydaf said:


> Scenario I
> However this person does not change the water on a regular basis and tries to create a blackwater setup with all kinds of leaf litter, driftwood, peat and tannins. All these acids accumulate and manage to lower the pH to 6.4. The KH remains the same, 9, so according to the chart this person now has ~CO2 107ppm. Poor fish must be gasping plants growing like crazy, though no CO2 was added. Nothing like this, in this setup the %carbonic acid which lowered the pH is minimal, the chart is totally inaccurate and useless.


In this situation, KH only remains the same if the test kit measures total alkalinity, rather then only the Karbonate hardness portion of total alkalinity. Tannis _will_ lower KH, since tannis _will_ convert carbonate and bicarbonate (Karbonate hardness) to carbonic acid.

In this situation, carbonic acid levels increase, however, the level of carbon content in the water has remained the same, since there was no net increase in carbon content, mealy a conversion between the carbon species

H2CO3 <--> HCO3 <--> CO3.

Notice how the carbon portion of the three species is identical (CO3), simply the number of attached H+ ions changes.

So even though the carbonic acid (H2CO3) level increases, any increase in CO2 dissolved in the water is minimal, since the carbon levels have essentially remained the same.

So really, it's not that the chart is inaccurate and useless, simply people do not fully understand the chemical process involved, and are misled by manufactures that test kits measure Karbonate hardness, when they do not.



dukydaf said:


> Scenario II
> Assume the KH=9 is the actual carbonate hardness. This person starts to add CO2 and cranks it too high. Now only dissolved CO2 is responsible for the change in pH. The pH drops to the same 6.4. Otos jump out of the aquarium and the Betta is happy that it belongs to Osphronemidae family.


Where we inject CO2 into the water column, only a small portion of CO2 converts to carbonic acid, so KH doesn't change because the increase in carbon content remains as CO2 molecules in the water. Since the carbon content from bicarbonate and carbonate (KH) hasn't changed (only a small portion is converted to carbonic acid), the total content of carbon in the water increases due to increased CO2 molecules in the water.

In this scenario, simple agitation of the waters surface will blow off the excess (in the form of CO2) carbon content, whereas in scenario I above, surface agitation won't change the carbon related content, because that carbon content is almost entirely due to ions in the water, not gas.


So............................as you rightly point out, using the chart to determine CO2 levels in a non injected tank is pretty much pointless. I just have this bad habit of throwing chemistry at everything. :icon_roll


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## dukydaf (Dec 27, 2004)

Audionut said:


> So really, it's not that the chart is inaccurate and useless, simply people do not fully understand the chemical process involved, and are misled by manufactures that test kits measure Karbonate hardness, when they do not.


Agreed, but in the case presented the chart alone is also misleading. People need to see this chart and read the text associated with it, such as presented by @plantbrain here: CO2/pH/KH table - Aquarium Plants - Barr Report





Audionut said:


> Where we inject CO2 into the water column, only a small portion of CO2 converts to carbonic acid, so KH doesn't change because the increase in carbon content remains as CO2 molecules in the water. Since the carbon content from bicarbonate and carbonate (KH) hasn't changed (only a small portion is converted to carbonic acid), the total content of carbon in the water increases due to increased CO2 molecules in the water.


But the unconverted portion of CO2 does not effect the pH. Though important for plants and taken into account in the ppm shown, we can ignore it when we talk about pH change.

Carbonic acid is a volatile acid but it is responsible for the pH change (not free CO2 as it as no H+). I think these are the complete ecuations

CO2 + H2O <--> H2CO3<-->HCO3(-) + H(+)
HCO3(-)<-->CO3(2-) + H(+)



Audionut said:


> So............................as you rightly point out, using the chart to determine CO2 levels in a non injected tank is pretty much pointless. I just have this bad habit of throwing chemistry at everything. :icon_roll


At this point I feel we both argue saying the same thing. I think we made our point. Sir,I always welcome a well argumented discussion, that it is chemistry is a bonus. 

Let us make a cup of tea, release some CO2 and watch the plants grow.


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## Zorfox (Jun 24, 2012)

I feel the OP is under the impression that CO2 will reduce the PH therefore, providing the proper PH for the fish. I think Diana gave some pretty good advice in that GH is more important than the actual PH. But why? And why will the PH reduction from CO2 not create a better environment for soft water fish?

To understand this we need to understand a system the fish have called osmoregulation. Basically, this system regulates the flow of water and minerals in and out of the fish’s body. This flow controls the osmolarity of the body which is the balance of minerals and water. Human’s get their minerals and water by eating them. However, fish can extract minerals and of course water directly from its environment.

There are two types of osmoregulation. Osmoconformers keep their body osmolarity in balance with their environment. Saltwater fish are typically osmoconformers. They “conform” to the environment.

Osmoregulators maintain a constant body osmolarity despite their environment. Freshwater fish are osmoregulators. They “regulate” their body osmolarity to a constant state despite the environment much like humans regulate their body temperature.

The amount of minerals in the water affects the PH. So when we say this is soft water what we are saying is that there is a low mineral content. Having low mineral content also means that the PH will typically be lower. The opposite is true for hard water systems.

Hopefully, it makes sense that fish are more sensitive to the mineral content of the water than the PH alone. Adding CO2 adds no minerals whatsoever. The PH fluctuations caused by CO2 are therefore insignificant in the whole scheme of things. 

Fish in CO2 tanks fare well despite huge PH fluctuations when we stop and add CO2. The same cannot be said when we alter mineral content enough to alter the PH an equal amount in a short time. 

So essentially, let your PH end up where it may and instead focus on GH and to a lesser degree KH for happy fish. You can spend all your time chasing a desired PH with chemicals but the fish are more interested in a stable mineral content than anything. Osmoregulation takes a fair amount of energy. Provide proper mineral content for your species and that energy can be used for other systems.


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## Audionut (Apr 24, 2015)

dukydaf said:


> But the unconverted portion of CO2 does not effect the pH............................Carbonic acid is a volatile acid but it is responsible for the pH change (not free CO2 as it as no H+)


But the converted portion of CO2 does affect pH. 

That increased CO2 levels increase carbonic acid, which then affects pH, doesn't rule out the role that CO2 plays in this shift of the chemical species. The primary affect of CO2 injection is increased CO2 molecules in the water, the secondary effect is the increase in carbonic acid which affects pH, _without a reduction in carbonate *sources*_.

In scenario I, CO2 plays no role what so ever. Instead, the sole cause of pH change is through the increase or decrease of carbonate sources. CO2 doesn't do anything, it simply stays at the equilibrium point with the atmosphere.

pH is only an indicator. It don't describe the concentrations of the chemical species, only the equilibrium point between the chemical species. For the same pH, we could have no (bi)carbonate, and little CO2, or lots of (bi)carbonate and lots of CO2.

https://en.wikipedia.org/wiki/Bjerrum_plot



dukydaf said:


> At this point I feel we both argue saying the same thing. I think we made our point. Sir,I always welcome a well argumented discussion, that it is chemistry is a bonus.


I think we are too, one of us is just pedantic, and it's not you. 
Thanks, discussion helps to refine knowledge.


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## plantbrain (Dec 15, 2003)

Measure the KH well, Lamotte are good. 
But do not assume the KH is absolute and correct, use that and the starting pH as a "relative" measure. 

Say the test says pH 7.0 with no CO2 gas added for 2 days or so.......and a KH of say 1 dKH.
Add enough CO2 to knock the pH down to say 6.0, you have about 30 ppm at that point. 
Now check that and compare with the pH/KH/CO2 table.

If it's very close or right on, then the KH is likely mostly bicarbonate and you can assume the table to a pretty good measure for you.

This tends to work with lower KH tap waters.

Now if you have a pH of 8.2 and a KH of 12, adding enough CO2 to get to 7.2 should add about the same CO2 ppm's. 
But this might not match with the pH/KH table so much, but you still can assume it's relative based on the starting initial degassed pH. 

Chemically, I think folks just should be aware that only 1 out of 400 molecules will form the acid base reactions........that detail throws a lot of chemist who think they are trying to help out. 
It would be great if we could just use CO2 gas to lower our KH. 
The remainder of the 399 CO2 molecules form CO2[aq]. 

As you add more and more CO2 gas(say from 50 ppm to say 100 ppm)..........for each 400 molecules you add, 1 will form carbonic acid, so the pH will drop, at least till you hit the end point for the acid. 
But those other 399 per unit? They are CO2[aq].

If you think about it, it should make sense. It's one of those issues that have logn caused issues for folks in the hobby.
But, adding enough CO2, how to do and measure it etc, those are pretty well understood. 

Focus there, trimming, water changes, clean filters etc. Which means I should go and take my own Advice eh? haha


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## VikeMan (Oct 13, 2015)

So... my water report has...
Bicarbonate HC03: 108 ppm
Total Alkalinity CaCO3: 88 ppm

Converting Bicarbonate HCO3 to Bicarbonate "as CaCO3":...
108 / 1.22 = 88.5 ppm

I think this is telling me that all of my total alkalinity is coming from Bicarbonate. Correct?


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## Audionut (Apr 24, 2015)

plantbrain said:


> It would be great if we could just use CO2 gas to lower our KH.


This is intriguing. One could say an advantage of CO2 is that it increases H+ ions, and hence changes balance, and pH, without affecting KH.

I would appreciate it if you could expand this point further.



VikeMan said:


> I think this is telling me that all of my total alkalinity is coming from Bicarbonate. Correct?


That is what it says.


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## VikeMan (Oct 13, 2015)

Okay, re-reading the whole thread is giving me a headache, but I will remain undaunted...

With a true Bicarbonate dKH of 5, and a pH of 7.3, how much CO2 would I have in the water _at_ _equilibrium_? Is it 8 ppm per the chart? Or is the chart irrelevant?


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## Audionut (Apr 24, 2015)

Welcome to my world. :tongue:

People who use the CO2 chart, admit that there is an error value.

I've yet to meet anyone who describes error with Henry's law, which in a nutshell says, "gases in an aqueous solution will always want to maintain equilibrium with the partial pressure of the gas in the surrounding atmosphere".

It is generally accepted that this equilibrium point is 3ppm of CO2 in the solution. In other words, when all else fails in a non-injected tank, assume you have 3ppm.


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## VikeMan (Oct 13, 2015)

Audionut said:


> It is generally accepted that this equilibrium point is 3ppm of CO2 in the solution. In other words, when all else fails in a non-injected tank, assume you have 3ppm.



Thanks! So, if I were to dissolve CO2 until the pH dropped from 7.3 to 6.3, would I have the 30 ppm of CO2 some have mentioned for a one point drop in pH, or would I have something like 76 ppm per the chart? Seems like a big difference.


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## Audionut (Apr 24, 2015)

I would lean more towards 30ppm then 76ppm.

Consider this observation from @Hoppy. The last paragraph is the important part imo.



Hoppy said:


> Knowing that the equilibrium concentration of CO2 in water exposed to the atmosphere is "about 3 ppm" doesn't really let us accurately determine how much CO2 we have in the water when we add CO2 to our tank. "About 3 ppm" might be 2.2 to 3.8, for example. A pH drop of 1.0, means we increased the amount of CO2 in the water by a factor of 10 (10 to the change in pH power) But that would mean our water has 22 to 38 ppm of CO2 - not very accurate. But, we can't measure pH much more accurately than +/- 0.2, so our 1 pH drop might be as little at 0.8 or as much as 1.2, and 10 to those powers is 6.3 and 15.8, so the real range of CO2 concentrations is 6.3 x 2.2 = 14ppm to 15.8 x 3.8 =60ppm which is now a useless measurement. Granted, you can possibly do a good enough job calibrating your pH probe so the accuracy is better than +/-0.2, but you will always have an error range greater than zero.
> 
> I think it is best to just assume we can't measure the ppm of CO2 in the water. What we can do is use a drop checker to tell us that we do have a significant amount of added CO2 in the water (we aren't just leaking the CO2 or degassing it rapidly). Then we can slightly increase the bubble rate of the CO2 and watch the fish and plants. We can repeat that until either the plants don't grow any better with the last change, or the fish are showing too much misery from the CO2. Back off the bubble rate to the previous rate and that is the optimum amount of CO2 for your tank, with your plant load, your lights, your oxygenation of the tank water, your fish, etc.


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## plantbrain (Dec 15, 2003)

Audionut said:


> This is intriguing. One could say an advantage of CO2 is that it increases H+ ions, and hence changes balance, and pH, without affecting KH.
> 
> I would appreciate it if you could expand this point further.


While it would be great, it just does not happen in any significant amount, some chemist seem to have issues with this when this is pointed out, (Observational facts, horrid stuff I know).
Only 0.25% changes, which is to say, not very much at all. 
And that's all it can do. 

That is measureable and how they got the ratio of about 400:1 remain as CO2 [aq], at least with marine seawater, I think it's very close with most freshwaters as well. You can also determine it a few ways with the carbonic fraction and then each method should match up well if the methods used, were done well. 

Not a bad chemistry assignment for a majors student. 

A hand measure for the entire Bjerrum plot was my torture. Did not miss doing that any.

Bump:


VikeMan said:


> Thanks! So, if I were to dissolve CO2 until the pH dropped from 7.3 to 6.3, would I have the 30 ppm of CO2 some have mentioned for a one point drop in pH, or would I have something like 76 ppm per the chart? Seems like a big difference.


If the water is fully degassed, there is no film on the water's surface(sometimes we cannot see it, other times, you can) etc, after 1-2 full days.......then yes, and the *relative* pH drop should add about 27ppm of CO2 additionally to the water if you use only CO2 gas to lower pH.


If you use say 1.2 pH drop, then it'll be about 45ppm more.

I tend to have 45-60ppm ranges in my tanks. 
But then again, I also have wet/drys and high O2.
So respiration for fish and shrimp, bacteria etc, is optimal, eg, they can tolerate more CO2 for those 7-8 hours I add it when the lights are on. 

And the other 16-17 hours they have 100% O2 levels or more with rapid removal of the higher CO2. 

Canister do not do that.

They still work well, but, the fish will have more issues as you adjust CO2 higher. No surprise there if you think about fish respiration(CO2 out, O2 in).

If you have high cO2 and low O2 in the tank, say 5ppm O2 vs say 8 ppm O2, and the same CO2, say 40 ppm, then the fish will be under much more stress with the lower O2.


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## Audionut (Apr 24, 2015)

Appolgies Tom, but I still don't understand why it would be good if CO2 changed KH.



plantbrain said:


> A hand measure for the entire Bjerrum plot was my torture. Did not miss doing that any.


Heh. I understand the principle of a Bjerrum plot, but I suck at maths that involves much more then addition or subtraction. Should have paid more attention in school back when I thought I knew everything.


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## plantbrain (Dec 15, 2003)

Then folks would not need an RO unit to soften their water. 

CO2 gas would acidify and remove it for us, but that does not happen.


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## Audionut (Apr 24, 2015)

Oh, guess they haven't heard of a calcium reactor!


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## mistergreen (Dec 9, 2006)

You can measure CO2 with a CO2 sensor 
But the price range is out of most hobbyist's budget and isn't needed with careful observation.


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