# Limited affect of CO2 In Hard Water ?



## Blueskiesdz (17 Oct 2015)

I'm tapping into your infinite knowledge please as I've probably received incorrect advice from my local fish shop. 

I've read and learned from this site for weeks before setting the tank up but the chap in the shop who's very confident has confused me some what !

Recently set up 36x18x18, 2 HO T5, Eheim Prof 350 and running CO2. I've recently planted it out with Tropica soil and I've noticed it takes an age (3 hrs +) for the CO2 drop checker to turn green (4 bps). The return is one end of the tank in a central position and the atomiser is low down in the opposite end. 

The chap in the fish shop told me the checker will struggle to reach the optimal level as the water is so hard in Leicesteshire and I should be using RO water. 

I personally think the checker is not going green due to poor water movement with only one source of movement maybe ? I really don't want to be using RO water again after years of using it in my reef tank which is now gone ! I was hoping just to use tap water via a mixer tap set at the right temperature and adding dechlorinator. Obviously I'll take advise and act accordingly if I'm asking for trouble. 

I've tested the ph in my tank and its  7 and 7.4 from the tap. I'm unsure the KH though. A silly question but will the KH change the more days c02 is injected into it ? 

Please someone give me the RIGHT advise.


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## Manuel Arias (17 Oct 2015)

Hi Blueskiesdz,

Well, several points from your comments:

-Lack of movement in the water: It is unlikely with the filter you have. Water mixing is not going to be the problem. The CO2 diffusor, however, it can have impact if generates too big bubbles, reducing the efficiency of your CO2 injection.
-Hardness in the water: It can be a problem. pH is not enough information to see this, as pH is affected by many factors and components in the water. You have to check kH to know where your levels of carbonates is.

The kH plays a main role in the amount of CO2 soluble in water in addition to the pH, so you have to control both to understand what is going on in your aquarium. Additionally, the temperature in the tank will also have a role into this. Note that the solubility of gases in water highly depends on the temperature, but in the usual scales this has a minor impact than the relationship between pH and kH. However, if you have 28ºC for example, it will be easily a significant variation respect to the solubility you get at 25ºC. 

For instance, you can find some ideas about this, in the tables representing this relationship and the maximum solubility of CO2 in the water:

Example here: http://www.theaquariumwiki.com/images/d/da/CO2_Graph.gif

So, in essence, if you have a pH of 7.2-7.4 and a high dkH, what is happening you is that your levels of CO2 are not high enough to trigger the change of colour in your indicator. However, it is not meaning that you have no the right levels of CO2. So, I would recommend to use this sort of table and measurements of kH and pH to see where are your maximums, in addition to the information of the indicator.

Other option is to check the water you use in the indicator. Usually, the CO2 indicators work by creating a small chamber of air between the indicator mix and the water of the aquarium. That works as follows: The aire acts as a gas exchanger between both liquids, so the concentration of CO2 in the chamber will depend on the concentration of CO2 in the tank. On the other side, there is a communication of CO2 between the indicator liquid and the chamber too. However, as the relative volumes are very different (volume of the tank vs volume of indicator liquid) the concentration in the chamber will depend only of the concentration in the tank. Hence, the flow of CO2 to the indicator liquid is just related to this one, and then the pH of the mix , which is the triggering factor of the colour, it depends only of the composition of the water in the indicator mix and the CO2 levels in the tank. If you are using water from the tank to prepare your mix, and the water has a high kH, then you are buffering the mix, i.e. increasing the maximum solubility of CO2 and then, having difficulties to impact the pH and see the change. You can see this by temporarily changing the water in the indicator by RO water, or a mix of RO and water in the tank. Note that most of these commercial indicators are calibrated to work at a pH of 6.8-7.

And finally, just a short explanation of what is happening: CO2 dissolves in water becoming in great proportion into HCO3- by dissociating a molecule of water into a proton (H+) and an hydroxile ion (OH-) and taking the hydroxyle to become in HCO3-. The reaction frees a proton in the water, making it more acid, reason why the pH reduces when injecting CO2 in the tank. HCO3- can also release another proton and become in CO3(2-), but due to the CO3 not being very soluble, this reaction happens in much lower proportion than the solution of CO2 in the water. The levels of CO3(2-) and HCO3- are then also in equilibrium. This implies that when you increment your kH in the water (that shows you basically your levels of CO3(2-) in it) you are pushing the equilibrium towards the formation of HCO3(-) from the levels of CO3(2-). As this reverse reaction takes one proton from the water, it increases the pH. This is what is called a buffer effect. The impact of this consist on when you add CO2, you are increasing your HCO3(-) concentration, but the acidification caused by it is hidden by the buffer effect of the CO3(2-), which keeps stable your pH. As the pH does not change, or change just a little, then your indicator mix will no reflect the increments of CO2 in the water, and then you are forced to provide higher dosis of CO2 to force any change of pH in the water. That is why I think part of the problem can be with the indicator mix you use.

I hope this help.

Cheers,


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## Tim Harrison (17 Oct 2015)

Short answer - ignore the dude in the LFS. I've always lived in very hard water areas and had no problems. 
It takes time to find the right balance, just keep playing with the CO a la Clive's tutorial http://www.ukaps.org/forum/threads/co2-measurement-using-a-drop-checker.467/ especially read the bit about Zeroing in on 30 ppm. 
It can take that long to get the lime green but if there are no critters to gas it makes it easier to experiment and find the optimum.
And don't get fixated on the bubble rate it's pretty meaningless, it's just an abstract refernece for you to help zero the CO2 in...


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## Blueskiesdz (17 Oct 2015)

Manuel thank you so much for taking the time to write such a detailed response. If im honest some parts are way above me at this stage in the game ! I'm a little embarrassed after so many years reef keeping that I can be so much of a beginner again ! It's good to try something new which is why I'm here.

In relation to water movement I was under the impression that the plants in the tank need to be 'swaying in the breeze' ? The return is hitting the far side and there is plenty of movement but on the other side nearest the return very little. My indicator is over near the return and I purchased it and the solution. I also have checked th temperature and it was 27 so I've adjusted down to 25.

So if there is a high dkH then the co2 even at the correct flow will show an incorrect reading on the indicator ?  So I suppose it would be quite feasible to keep increasing the co2 to get the right colour but in turn wiping out the fish etc in the tank ? In this situation would it be wise just to scrap the indicator and use a dkH and ph test kit and refer to the graph you kindly supplied ?


Troi thanks for your input mate much appreciated. With reef I know a reasonable amount so I don't generally listen to 'I heard it from a man down the pub' type advice. I'm just new to planted tanks and perhaps like in any situation when your looking for advice or open to advice people can give you their suggestions (which are definitely right!) As I said above to Manuel could I go overboard on the co2 just to get the green colour in the indicator ?  The mist/fog  from the atomiser also from co2 art is quite something and I'm unsure whether I should turn the co2 up anymore to get the right colour. This is why I mentioned flow as I presumed the fine bubbles needed to flow around the tank to get to all the plants.

Bear with me I'm learning


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## Tim Harrison (17 Oct 2015)

No worries...and so am I - still learning that is


Blueskiesdz said:


> So if there is a high dkH then the co2 even at the correct flow will show an incorrect reading on the indicator ? So I suppose it would be quite feasible to keep increasing the co2 to get the right colour but in turn wiping out the fish etc in the tank ? In this situation would it be wise just to scrap the indicator and use a dkH and ph test kit and refer to the graph you kindly supplied ?


A drop checker is a bit of a blunt instrument, just as most hobby grade test kits are...but the short answer to your question is no.
A drop checker filled with 4dkH solution is still the best way of measuring CO2 conc. there is very little chance of wiping out your critters if it's used correctly - if there were, none of us would be using them

If you're worried about your flow try pipetting some non-toxic coloured liquid in to the tank and take a look at the flow dynamics.
Not sure about the turnover of your filter but the standard is around 10x tank volume per hour. So if your tank is 100 litres the filter needs to turnover 1000l/h. However, its the flow that's important so if you're short you can always just add a powerhead.


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## Manuel Arias (17 Oct 2015)

Hi Blueskiesdz,

Well, yes. I tend to be too much technical, but I always believe that understanding the principia behind the things help a lot in doing the right things.  I am glad that the answer could help you.

Regarding the question of the filter, this is quite easy to see. I assume you gave the dimensions of your tank in inches, so that goes for 183 litres tank, approximately. The Eheim professional 350 has a flow rate of 1050 l/h so the number of times per hour the whole volumen of the tank tank pass by the filter is around 6. ADA systems, that I do not believe we can complain about, have rates for that size of tank between 300 l/h for the 600 Series (up to 90x45x45cm tank) and 960/l for the 1200 Series (starting at 90x45x60cm tanks and up to 120x45x60cm), so clearly your rate is good enough. In fact, too much flow can cause mechanical stress to the plants, and not all the species will tolerate it well. As fishes, different plants will grow better under specific flow conditions, so I really doubt the rate you have is the problem in your CO2 levels. If you are not very happy in how the CO2 is dissolved and spread in the tank, you can go for a better diffusor, or even to an in-line injector, which usually optimizes the dissolution of the CO2 bubbles and also making them smaller.



Blueskiesdz said:


> So if there is a high dkH then the co2 even at the correct flow will show an incorrect reading on the indicator ? So I suppose it would be quite feasible to keep increasing the co2 to get the right colour but in turn wiping out the fish etc in the tank ? In this situation would it be wise just to scrap the indicator and use a dkH and ph test kit and refer to the graph you kindly supplied ?





Troi said:


> A drop checker is a bit of a blunt instrument, just as most hobby grade test kits are...but the short answer to your question is no.
> A drop checker filled with 4dkH solution is still the best way of measuring CO2 conc. there is very little chance of wiping out your critters if it's used correctly - if there were, none of us would be using them



And regarding the drop checker, the right answer is that works meanwhile you have some considerations. As mentioned by Troi, and also pointed out by me in the first answer I gave, the kH levels in the drop checker are rather important to make sure the readings in the checker are valid. If you have a high dkH in the drop checker, then you will face problems to get the right readings. As Tori proposes, you can made your own water for the drop checker and fix it to dkH of 4, which is a quite soft water, so definitively, if you have hard water in the tap, tap/aquarium water will not make a good solution, I think. And regarding your question, if the dkH in the drop checker is too high, then you could have the right levels of CO2 but not seeing it in the drop checker. As commented also before, the drop checkers only reacts to pH changes in the mix of water and indicator in the drop checker, so if you are buffering the mix by using hard water, then it will not give you the right readings.

Of course, this does not mean that the drop checker is useless, but I think that interpreting what it is telling is not just a matter of expecting the right colour. They get green colour only when the pH goes between 6.8-7 inside the drop checker mix. Over 7 you will find it blue, and below 6.8 you will find it yellow. For instance, if you pay attention to the table I put before, and imagine a dkH of 8, the amount of CO2 that has to dissolve to see impact in the drop checker is about 24 mg/l. However, with a dkH of 4, the amount of CO2 becoming green the indicator is of 12 mg/l. So you can see the impact of what I am saying. This is still safe for your plants ad fishes, but explains why you have to inject a lot more CO2 in the tank to get the right levels at high dkHs than a lower dkHs.

Cheers,


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## MirandaB (17 Oct 2015)

Great reply Manuel although I may need to read it a few times for all the tech stuff to sink in


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## Manuel Arias (17 Oct 2015)

MirandaB said:


> Great reply Manuel although I may need to read it a few times for all the tech stuff to sink in



 Thanks! Well, quite technical true be said. In any case, a thorough explanation is found in the link provided by Troi in this thread, if that heps, as there is also explained what I am saying in plain words, more or less. I can, however, provide any clarification if required, so ask me if needed. 

Cheers,


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## Blueskiesdz (21 Oct 2015)

I've just completed a dkH and it 4 and the ph is 6.2. Looking at the above table it shows double the required levels ! The confusing part for me is that the drop checker is still showing a shade of blue !! The solution is from co2 Art and is new so I would not have thought that's the issue. My bubble counter must be 8 drops a seconds. 

 Which result would be the reliable reading ?


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## alto (21 Oct 2015)

Don't forget that the drop checker has a delayed response time of at least 30 min (some posters report an hour or more ... )

In terms of flow/distribution, you can play with positioning of the inlet & outlet

eg I have mine at opposite ends of the tank, I use a single spray bar (it came with the filter kit  ) pointing the length of the tank so there's not a chance the stream will get anywhere near the opposite side of the tank ... but it fits nicely   (& provides more even flow than alternate positions)

I have an Eheim Prof 250 on a similar sized tank (90cm x 45cm x 52 cm internal dimensions) & there is some movement of plants following the outflow from the spray bar, the C helferi  planted near the intake has leaves flowing back in the direction of the spray bar so I can easily visualize the circular flow path (coincidence) ...
there isn't much visible current movement at the substrate level directly near the spray bar but I have the CO2 diffuser situated there & can visualize the microbubbles that rise ~ halfway, then are dispersed into the water column (none rise to the top of the tank in a direct path from the diffuser)  
I have very soft water, kH 0-1, GH 1-2, acidic pH (except when the water board is playing with additions)) so I can gas my fish very easily - that bicarbonate equilibrium in water that Manuel is referring, in soft water, it's the dominant pH effect.

So your local fish shop chap is correct - in a practical sense, it's much easier to get desired CO2 levels in RO water 
- but you'd also better stay on top of your nutrient dosing, as there's none otherwise, hence the recommendation to mix tap & RO water if you decide to go this route ... which I wouldn't!
In the end, there are always some plants that do _better_ in soft or hard water, though most can be cultivated in either.

Check kH & GH of your water, also pH out of tap vs standing overnight, you can get a detailed water analysis through your water supplier - information is often online already.


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## Blueskiesdz (21 Oct 2015)

Hi Alto thanks for your reply.

So would I need to turn the co2 up even higher to get the drop checker green at the point the lights come on ? My confusion is the test kits results looking on the graph show double at that point. So the result in the checker was from an hr+ ago so I'm thinking I'd need to turn the co2 up even more to get it right at lights on. 

My tap water ph is 7.6 and kh7. 

I've also noticed some of the plants are yellowing which im guessing is from insufficient nutrients. Would high co2 cause any issues like this ? Also the crypts are melting but again from reading on ukaps this happens regularly I think ? 

With regards to flow I have the return one side and the out flow the same side  of the tank just opposite corner. The diffuser is at the far side low down where the water will be initially hitting after being filtered. 

I just don't want to keep turning up the co2 un necessarily. At the moment there's no fish etc so no suffering.


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## alto (21 Oct 2015)

I've not looked at the specific linked graphs but by convention they are generated for systems in which bicarbonate in the dominant buffer with limited contribution from other buffering systems which may occur in water, hence my suggestion to check your local water analysis.

You can also try running lower levels of CO2 24/7, eg begin with 2bps & see how plants respond, you may still obtain best result by increasing CO2 to 4bps at 1-2h before lights come on, then reduce CO back to 2 bps 1-2 h before lights off ...
in other words, let your plants tell you what works best/better rather than going by chart/drop checker.

Do you have surface scum/film? this will greatly impact gas exchange with atmosphere (your source of oxygen dissolved in water, unless you have a venture device on your filter or a sump etc)

As to yellowing, how long has tank been set up, which plants etc, etc

Most plants are grown emerse, than adapt to submerse culture in tanks ... depending on various factors, the emerse leaves may remain intact for some time or show almost immediate "_damage_"  (often related to shipping etc trauma rather than immediate response to your particular submerse conditions, also factored by leaf type/species).

Crypt melt is often just an indication of _change_ in the environment (they melt in the wild too)


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## Blueskiesdz (21 Oct 2015)

The tank has been set up one week  There is no surface scum as I have a surface skinner attached to my filter. 

I'm unsure the type of plants but there's slight yellowing on most of them. I have the lights on for 5 hrs with the co2 coming on two hrs before. Would it be a good idea to reduce the co2 but increase the time it comes on before lights on ?


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## alto (21 Oct 2015)

I'd begin conservative

1) leave same CO2 levels surrounding your current light period

2) run reduced CO2 24/7 OR begin CO2 4 hours prior to lights & stop CO2 1 hour after lights (again this goes back to how much ambient light tank receives)
The CO2 schedule is about getting/maintaining sufficient dissolved CO2 in the water column during hours of plant respiration not just during hours of artificial light.

3) if you see consistent improvement over the next 1-2 weeks, then this may be the path to follow (or some version of it anyway)

Try to change 1 thing at a time, then wait.

At this point, we're focusing on CO2 levels & assuming they are insufficient.
You can try increasing CO2 flow into the tank, try adjusting CO2 dissolution into the water column, look at CO distribution in the water column (though if most plants in various areas of the tank show similar symptoms, distribution is an unlikely culprit) 

How much growth are you seeing?
Again it's helpful if your plants are known identity - go back to seller & request id's
You can also post detailed photos & get some decent guesses here.

Some queries:
Tropica soil - powder or regular?
Tropica Growth Substrate?
Fertilizers?
Plant nursery? 
Water change details?


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## Blueskiesdz (21 Oct 2015)

I'm using tropica soil and easylife profito ferts. Water change every 3 days approx 40% each time. Plants are mainly tropica but some from aqua flora and flora media. The ones which don't appear to be doing to well are -

Pogostemon helferi melting leaves

Microsorum pteropus trident yellowing

Juncus reopens leaves yellowing

Pogostemon erectus leaves turning black

Growth in most plants is good just some appear to be suffering with yellow and black patches and this is all around the tank. Lights on 5 hrs and there's not to much ambient light.


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## alto (23 Oct 2015)

I'm not familiar with this fertilizer range but did find this

_ProFito does not contain nitrate or phosphate_

This is unlikely an issue in the early stages when plants are mostly growing/adjusting from reserves but you may want to look at this later, some tap water contains sufficient N & P, heavy fish loads/feeding can supply N & P ...

Only the M pteropterus stands out on that list, it's usually a very sturdy plant, whereas similar observations for the others you mention is not uncommon.

Have you looked through Mark Evans journals - lots of excellent photographs in his journals, take note of the light levels, especially distance from the tank to the luminaire - it's quite possible that you are still CO2 deficient for the light levels, I'd not change your 5h photoperiod but would likely raise the lights (assuming they are fairly close to the tank surface) and increase CO2.
Once plants are established & there's little signs of algae, begin to back off on the CO2 gradually 

Also remove damaged/melting leaves as they contribute little to plant health, most feel they act more as a drain on plant resources - certainly they affect the integrity of the plant's barriers to external environs.

Unfortunately there is no guaranteed RIGHT advice, only guesses based upon what you report, the more detail you're able to include, the more educated the guesses coming in ... photos are often very helpful in triggering more guesses & more insightful guesses


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## Blueskiesdz (23 Oct 2015)

Alto thank you as ever for taking the time to give me such great advice. The co2 is now spot on after I started the co2 3 hrs instead of 2 and turned it down to 3 drops a minute. I think now a week later the tank is starting to balance out a little bit more.


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## ceg4048 (24 Oct 2015)

Manuel Arias said:


> The kH plays a main role in the amount of CO2 soluble in water in addition to the pH


This is not true at all. There is no relationship between the KH or pH of a water sample and the solubility of CO2 in that water.
The solubility of CO2, or ANY gas is a function of pressure, temperature and to some extent, salinity. No other parameters or factors are relevant.



Troi said:


> Short answer - ignore the dude in the LFS.


 Words to live by. 

Cheers,


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## Manuel Arias (24 Oct 2015)

ceg4048 said:


> This is not true at all. There is no relationship between the KH or pH of a water sample and the solubility of CO2 in that water.
> The solubility of CO2, or ANY gas is a function of pressure, temperature and to some extent, salinity. No other parameters or factors are relevant.



Apologies for debating you all the time, but here, again, you are mistaken. In general the solution of gases, by physical laws, depends only, as you well said, pressure, temperature and some extent, salinity. However, this is only true under the point of view that the gas concentrations of water are not affected by external factors.
When you increment kH levels, you are incrementing carbonates levels, and I am sure we will agree into that. At the same time, carbonates act as a buffer element of the solution, hence, they have capability to capture protons from the water, avoiding the pH to change when adding an acid solution to the water. The carbonates then become in HCO3 when protons are released to water. On the other side, CO2 becomes in HCO3- when goes to solution, so the three species, CO2, HCO3- and CO3(2-) are in a chemical equilibrium that is triggered by the pH, as shown in the following figure:




 

As you can see, there is a relatinship between the chemical form of inorganic carbon and the pH, which also links to the kH, as this parameter represents the total content of carbonates. This mechanism can limit the pCO2 in water, as in fact, the reason why CO2 is so soluble in water is because becomes in HCO3- in most of the total flux of CO2, not being a gas anymore, and then allowing more flux of CO2 to the water until the point of saturation of HCO3-, and then starts the carbonates formation. This whole balance is controlled by pH. Hence, your assumption that CO2 only plays with the inert gas laws is not accurate. You can find some information about that here:

http://www.soest.hawaii.edu/oceanography/faculty/zeebe_files/Publications/ZeebeWolfEnclp07.pdf

And also explains why so many aquatic plants have adaptations to use HOC3- in addition of CO2, like the mechanism you explained some time ago:

http://www.ukaps.org/forum/threads/what-form-of-carbon-do-water-plants-use.26887/

And please, let me remark this: The only reason why we can increment the CO2 levels until such a point is because becomes in HCO3- which is no longer a gas, so leaves the games of the perfect and inert gas laws, allowing more injection.


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## fablau (8 Nov 2015)

alto said:


> The CO2 schedule is about getting/maintaining sufficient dissolved CO2 in the water column during hours of plant respiration not just during hours of artificial light.



Is that really true? For example, my lights turn on at 2pm, and my Co2 starts at 10am, but I have some ambient light around the tank between 7am and 10am... So, should I have my Co2 turn on at 6-7am??


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## alto (9 Nov 2015)

Depends on type of plants & how much ambient (sun) light tank receives .... you can often tell by looking at the plant leaves - are they already "day open" or still "night closed"  (some plants are much more obvious about this than others).
If your tank is running well with your current settings, I'd just leave them as is


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## dw1305 (9 Nov 2015)

Hi all, 





Manuel Arias said:


> As you can see, there is a relatinship between the chemical form of inorganic carbon and the pH, which also links to the kH, as this parameter represents the total content of carbonates. This mechanism can limit the pCO2 in water, as in fact, the reason why CO2 is so soluble in water is because becomes in HCO3- in most of the total flux of CO2, not being a gas anymore, and then allowing more flux of CO2 to the water until the point of saturation of HCO3-, and then starts the carbonates formation. This whole balance is controlled by pH. Hence, your assumption that CO2 only plays with the inert gas laws is not accurate.


 I think this is right when you don't add CO2, but I think it is different when you add CO2 .

Only a very small proportion of CO2 goes into solution as H2CO3, most of it remains as CO2 (gas), which is the "total carbon dioxide" figures on the left of the curve.

My understanding is that carbonate solubility (as HCO3-) is dependent upon the CO2 concentration, and the graphs are at equilibrium with  atmospheric CO2 levels (~400 ppm). 

If we continually add CO2, we can achieve much higher CO2 values, and we can <"estimate the level of dissolved CO2 from the pH drop for a known dKH value">. 

cheers Darrel


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## alto (9 Nov 2015)

dw1305 said:


> but I think it is different when you add CO2 .


When you alter the concentration of any component in an equilibrium, there will be an equilibrium shift


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## fablau (10 Nov 2015)

alto said:


> Depends on type of plants & how much ambient (sun) light tank receives .... you can often tell by looking at the plant leaves - are they already "day open" or still "night closed"  (some plants are much more obvious about this than others).
> If your tank is running well with your current settings, I'd just leave them as is



Plants are completely "open" at around 8am, and some of them really bend toward the near window, even though it is just indirect light.

My tank is ok, but not perfect. I still have some BBA around, and I 'd like to have pristine conditions. Maybe I should try to have more Co2 earlier in the day?


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## rebel (10 Nov 2015)

ceg4048 said:


> This is not true at all. There is no relationship between the KH or pH of a water sample and the solubility of CO2 in that water.
> The solubility of CO2, or ANY gas is a function of pressure, temperature and to some extent, salinity. No other parameters or factors are relevant.
> 
> Words to live by.
> ...


Agree. If you want CO2 in your water. just add it. If not enough, (use plants to judge), then add more. If too much, (judge using fish), then reduce. Ignore LFS. Show them your glorious planted tank.


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## alto (10 Nov 2015)

fablau said:


> Maybe I should try to have more Co2 earlier in the day?


Try this on days when you're able to monitor fish.
I tend to run low levels of CO2 24/7 (tap water is very soft with KH <1, GH<1 or occasionally 2), adjusting filter return to provide more surface agitation during dark/dim hours.


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## ceg4048 (16 Nov 2015)

Manuel Arias said:


> However, this is only true under the point of view that the gas concentrations of water are not affected by external factors.


The only external factor involved here is that we are increasing the partial pressure of CO2. The KH of the water cannot be considered an external factor because it is a fundamental property of the water. Increasing the carbonate concentration has the effect of buffering the water's pH against changes made by the liberated H+ as equation (2) moves to the right by virtue of the CO2 partial pressure increment introduced on the left by our injection.

The KH of the water has nothing to do with preventing CO2 solubility in that water. Given similar atmospheric conditions (pressure, temperature and salinity), around the world, CO2 dissolves equally well in all bodies of water, no matter their varying Carbonate content. The same amount of CO2 is dissolved in high carbonate Lake Tanganyika as in soft water Scottish streams because the atmosphere above the water is pushing the gasses into the water at similar rates (again, assuming temperatures are similar).

Where you are mistaken is that first, you have incorrectly assumed that 100% of the CO2 is involved in the Carbonic acid equilibrium equation, and that is simply not the case. It's exactly as Darrel has pointed out to you and it's as plain to see that they show that the percentage of CO2 that STAYS as CO2 is 99.85%. Only the 0.15% of the CO2 reacts to become Carbonic acid - and THAT is the percentage that becomes zero at pH 8 on your chart (and which is stated as 0.5% in the speciation discussion on your link).

The other issue that people who wave this chart around seem to conveniently forget, is that if you have water with a high natural pH then it can only be high pH because of high natural levels of carbonate/bicarbonate. So of course the percentage of Carbonic acid derived from the small percentage of CO2 reacting with the water compared to the highly buffered water will be very small.

Additionally, if this is an equilibrium equation, and if all the CO2 turned into something else, wouldn't the partial pressure of CO2 in the water then fall to zero? And wouldn't the atmosphere just above the water then push more CO2 into solution continuously? Also, if this is an equilibrium equation, and if the chemical conversion had indeed evacuated all the CO2 from the left side of the equation, wouldn't equilibrium then force the equation the other way, to the left. How can CO2 simply turn into something else and completely disappear if the equation is bi-directional?

The answer is: It doesn't disappear, no matter what the pH or KH because the solubility is a strict function of pressure and temperature and those parameters determine the equilibrium equation, they are not defeated by it, so CO2 will ALWAYS be present in the form of CO2 no matter what pH or KH.



Manuel Arias said:


> This mechanism can limit the pCO2 in water, as in fact, the reason why CO2 is so soluble in water is because becomes in HCO3- in most of the total flux of CO2, not being a gas anymore, and then allowing more flux of CO2 to the water until the point of saturation of HCO3-, and then starts the carbonates formation.


Again, incorrect. CO2 must first dissolve into water before any of these reactions occur. CO2 is very soluble in water simply because it has a moderately high solubility coefficient. Hydrogen Sulfide (2 times more soluble), Chlorine (3 times more soluble), Sulfur Dioxide (65 times more soluble), Ammonia (250 times more soluble), all have higher solubility coefficient that CO2 and they do not involve Carbonic acid formation.




Manuel Arias said:


> And also explains why so many aquatic plants have adaptations to use HOC3- in addition of CO2, like the mechanism you explained some time ago:
> 
> http://www.ukaps.org/forum/threads/what-form-of-carbon-do-water-plants-use.26887/


No this doesn't explain why plants use this Proton Pump strategy. Plant use the strategy because it works and the equilibrium equation is bi-directional. In many waters, the Carbonate content of the water is naturally due to the presence of dissolve carbonates while the CO2 availability is low due to many factors such as high temperature, poor flow, low gas diffusion rates, competition for CO2 by other plants and microbes and so forth.


Cheers,


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## hard determinist (19 Nov 2015)

ceg4048 said:


> CO2 will ALWAYS be present in the form of CO2 no matter what pH or KH


I'm afraid you forgot to take the TIC and degassing issue into account. It seems you make no distinction between a system with a constant TIC level vs. a system with artificially maintained (increased) TIC level. This is very important to distinguish when we speak about CO2 and its levels in water. When you add CO2 into water (from a cylinder), then you are increasing the TIC (= total inorganic carbon) level together with CO2 level, as more added CO2 adds to TIC also. So you have three main carbon "bodies" in water: [CO2] + [HCO3-] + [CO3--]. All these "bodies" make up the TIC content. So for example, under pH 6.5 and KH=4 the total inorganic carbon (TIC) level is 2.43 mM/L (this much carbon is there in the water). If you measure the HCO3- ions concentration and find out it's KH=4, it means that there is 1.43 mM/L HCO3- ions (or 87 ppm). Based on the equilibrium equation you can calculate the amount of the two other species: CO2 and CO3--. And you get 0.002 mM/L CO3-- (= 0 ppm), and 1.00 mM/L CO2 (= 45 ppm). Also, of this 1.00 mM/L CO2, a small fraction (about 0.2%) is there as H2CO3, of which small part dissociates into H+ ions increasing the total hydrogen ion concentration by 0.0003 mM/L H+. So under pH 6.5 you have 45 ppm [CO2] : 87 ppm [HCO3-] : 0 ppm [CO3--] in water. However, this state is not "natural". The water is oversaturated by CO2. Under normal (natural) conditions the CO2 level should be only about 0.4 to 0.6 ppm. So if you have such a tank (or river) with 45 ppm dissolved CO2, there must be some constant supply of CO2! Once you stop this supply, the CO2-oversaturated water will degas the excess CO2, and its level will balance out with its concentration in the air (400 ppm in the air = 0.4 ppm of dissolved CO2 in water). Due to this the TIC will drop, but the HCO3- concentration (= KH) will stay the same [unless we add some strong acid in there, or plants will deplete it during their photosynthesis]. So when you hypothesize about changing CO2 or HCO3- levels, don't forget to take into account the TIC also, and the rate of degassing. So there is (for sure) a strong correlation between the pH/KH and [CO2] in water! Because this correlation exists, you can calculate the total CO2 level in your tank based on the pH and KH values (when I'm speaking of "KH" here, I mean "HCO3-"). It's not true that only the small percentage (0.2%) becomes zero at pH 8. The chart shows the total [CO2], and not just the H2CO3 fraction! The total CO2 is however calculated based on this fraction (and its relationship or correlation to CO2/HCO3- and pH). So what happens when you lower the KH (HCO3- ions concentration), for example by adding a strong acid like HCl into water? The new H+ ions will reacts with HCO3- ions forming H2CO3 which quickly dissociates into CO2. So HCO3- ions are consumed up in this reaction (KH rapidly decreases), and CO2 molecules are created (total CO2 level is rapidly increasing). But under normal conditions, most of this newly created CO2 just quickly degas. So you won't be able to keep it in the water. How much HCO3- ions will be depleted and how much CO2 is created depends on the amount of added acid (H+ ions). If you have 1.43 mM/L (87 ppm) HCO3- ions in your water initially, and add 1.08 mM/L of HCl, you will end up with 0.35 mM/L (22 ppm) HCO3- and 1.08 mM/L H2CO3 (of which 0.25 mM/L = 11 ppm remains in water under pH 6.5 and continuous CO2 supply, and 0.83 mM/L = 37 ppm degas out of water). So if you had 2.43 mM/L TIC initially, you end up with just 0.6 mM/L TIC.

This matter is a really complicated one, and most confusion arises when someone who don't understand it well is playing on a guru.
No CO2 level above ~0.4 ppm will ever stay in water unless you continue to add more and more CO2 in there (thus overcoming the degassing rate)!
Once you increase the CO2 concentration in your water above this natural level, the H2CO3 concentration (and thus pH also) will adjust accordingly. And after you find out the HCO3- ions concentration in your water (aka "KH"), you can calculate the amount of total [CO2] in your water. This should be enough for most hobbyist to know.


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## dw1305 (19 Nov 2015)

Hi all, 
I'm not a CO2 user, but I think you are right, dissolved gases and gas exchange is generally a  subject that aquarists find quite confusing and complicated.  





hard determinist said:


> This matter is a really complicated one.......No CO2 level above ~0.4 ppm will ever stay in water unless you continue to add more and more CO2 in there (thus overcoming the degassing rate)!





hard determinist said:


> It seems you make no distinction between a system with a constant TIC level vs. a system with artificially maintained (increased) TIC level.


 I think this is the important point, the drop checker etc is just for water with an "_artificially maintained (increased) TIC level_" (TIC = total inorganic carbon). 

In this post I used the <"Bouncy Castle"> analogy to describe both the addition, and degassing, of CO2. I'm not sure if it  helps.

cheers Darrel


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## ceg4048 (19 Nov 2015)

Hello,
         At no point have I stated that there is no relationship between various forms of TIC. I also ignore off-gassing because it is not germane. The degassing rate is a separate issue. It does occur, is significant and can be taken into account from a practical level, but if you think that 0.4ppm is the limit then it's clear you have never used CO2. You can annihilate the tank inhabitants in short order with CO2 - and they die from CO2 poisoning, not from Carbonic acid poisoning.

Also, I think you have missed the point of my argument. What I am saying is that the relationship between CO2 content and pH/KH is based on the 0.15% of CO2 that reacts with the water. As more CO2 is added the equations are driven by that 0.15%, whose absolute value increases as the CO2 partial pressure increases. The remaining 99.85% of the CO2 remains as dissolved CO2.

Cheers,


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## hard determinist (19 Nov 2015)

ceg4048 said:


> Only the 0.15% of the CO2 reacts to become Carbonic acid - and THAT is the percentage that becomes zero at pH 8 on your chart


This statement proves you guity of being mistaken in your understanding of CO2 behaviour. At pH >8 you have (nearly) no dissolved ΣCO2 in water. But it's OK. I know that some people here (and elsewhere) are unerring. I don't correct you because of you (because I know it is pointless); I do it for the ones who wants to listen.


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## dw1305 (19 Nov 2015)

Hi all, 





hard determinist said:


> At pH >8 you have (nearly) no dissolved ΣCO2 in water.


We are just going around in circles here. When you add CO2 (so that the rate of addition exceeds the degassing rate) you push the HCO3- ~CO2 equilibrium towards CO2. 

More CO2 in the water = more H2CO3 and the additional H+ ion cause the pH to fall.  When you add 30ppm of CO2 you get a drop of about 1pH unit, it doesn't matter how much HCO3- you have (pH drop is independent of dKH). 

Calcium carbonate is insoluble in water, but in water with carbonates present the small amount of CO2 (that goes into solution as H2CO3) is in equilibrium with the HCO3- to give a stable value of ~pH8 at atmospheric CO2 levels (400ppm CO2) and standard barometric pressure (1013mb). Carbonic acid (H2CO3) and bi-carbonate (HCO3-) are the weak acid and weak base pair in carbonate buffering.

When we add CO2 above atmospheric levels we drive the H2CO3 ~ HCO3- equilibrium towards H2CO3. We know that pH is a ratio, and that an acid is defined as a H+ ion donor and we've added extra H+ (from H2CO3), so the pH falls. How much the pH falls depends upon the reserve of carbonate buffering, we usually measure this as "dKH". 

In a drop checker we have an air gap and use 4dKH solution with a narrow range pH indicator (bromothymol blue) to estimate the CO2 value from the dKH, pH, CO2 chart. The air gap means that the pH value (indicated by the change in colour of the pH indicator) is reliant on the CO2 degassing from the tank water, and then going back into solution as H2CO3 in the drop checker. The drop checker is isolated from both tank water (by the air gap) and atmosphere (it has a sealed top).

cheers Darrel


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