# CO2, pH and kH question, help needed :)



## brightstar123 (Oct 20, 2013)

Sorry to revisit this topic, just need some advice from an expert or two! It's our first planted tank, 580L, radion LED lighting, CO2 added etc, 70% planted, been running 5-6wks approx.
Here's what we have test-wise:
Tap water pH: 8
Tap water kH: approx 35.8ppm
Tank pH before CO2 comes on: 6.4
Tank pH after CO2 turns off: 6 or likely less (API kit doesn't show lower, need to buy a better one!)
Tank kH: between 17.9 and 35.8ppm 
Tank gH: approx 53.7.
Problems are that pH is a bit low to start off with, but there is lots of driftwood and aqua soil (although it has a gravel cap). So when we run the CO2 at a rate that causes good plant growth, pearling etc, the pH is a bit low for our rainbowfish perhaps? The rate of CO2 that causes the drop checker to change to lime green is at least 20bps and the drop checker only changes properly when our fish show slight signs of distress, so we had to dial it back a bit.
Do you think we need to increase kH to slightly increase our pH? Or should we not be too concerned about a pH possibly below 6 for most of the day? Plants are growing very well and no algae so far.
Should the drop checker go back to blue overnight? It's a bit annoying, I'm inclined to just watch the fish and plants and base the CO2 dosage on that! 
Any advice very welcome!


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## plantbrain (Dec 15, 2003)

Toss the drop checker, stick with pH and KH(these are the best relative measures). 

pH/KH will only either be close to the real value or OVER estimate the real ppm. So you think you have more CO2 ppm than you really do, this is true is certain types of tap water due to the water treatment processes, aquariums with soil, peat, lots of tannins etc.

So use the pH/KH as a target, then SLOWLY adjust by using the pH and dropping it using ONLY CO2 gas by say 0.05 units each 3-4 days and watch carefully the plant's new growth, the fish eating and general behavior. 

You do this process over say 1-2 months, you should be able to target the optimal CO2 addition rate for any aquarium. This rate will vary tank to tank.

A drop of a mere 0.4 pH is way too little for the CO2. My KH is roughly the same as yours, my pH's are in the 5.8's. 
Your tank might not be degassing much at night, or the soil and wood might be depressing the pH more than my tank etc.
Note, if you lower the pH with peat, say from 7.0 to 6.0, you do not magically get 30 ppm more CO2.
You ONLY get more CO2 by adding more CO2 gas.


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## dinnese (Feb 4, 2013)

When injecting CO2 into a planted tank, it is useful to know how much CO2 is actually getting dissolved into the water. Lucky for us, there is a simple way to calculate the CO2 level, based on the pH, and KH of the tank water. There is a fixed relationship between the pH, KH, and CO2 level. As you increase the amount of CO2 that is dissolved into the water, the pH will drop. And if you then stop adding CO2, the pH will climb as that extra CO2 is released from the water.

A little background science:
pH is a measure of the concentration of hydrogen ions [H+] in solution. pH is actually a logarithmic measure (-log[H+]). This formula has two implications. First, it means that the hydrogen ion concentration increases (thus the solution becomes more acidic) as the pH number decreases. Second, each time the pH is reduced by 1, the concentration of hydrogen ions increases by a factor of 10.

In more simple terms, the pH is simply a measure of the acidity or alkalinity of a solution. pH values from 0 to 7 are considered acids, and pH values from 7-14 are considered bases. 7 is neutral, neither an acid nor base.

As we add CO2 to water, it forms carbonic acid, which lowers the pH. The more CO2 that gets dissolved into the water, the lower the pH.

Working to raise the pH of the water is the KH. While KH refers to Carbonate Hardness, what is really measured by a standard KH test kit is really the buffering capacity. In "most" water sources, the buffering is provided by Carbonate. In that case, buffering capacity and KH are the same thing. Assuming a constant amount of CO2, a higher KH, will result in a higher pH.

Myth: A Low KH results in a larger pH swing when adding CO2.
Many people are under the mistaken impression that a low KH results in large pH swings when adding CO2, while raising the KH will result in smaller pH swings. This is not the case. The KH will move the start and end pH values, but the pH swing will be the same for a given level of CO2. You can see this in the chart below, or using the calculation:

Case 1: Assume a KH of 15 degrees, and a starting CO2 level of 4.5ppm, which would result in a pH of 8.0. If we then add CO2, to increase the CO2 level to 28ppm, that would drop the pH down to 7.2, for a pH shift of .8.
Case 2: Assume a KH of just 1.5 degrees, and a starting CO2 level of 4.5ppm, which would result in a pH of 7.0. If we then add CO2, to increase the CO2 level to 28ppm, that would drop the pH down to 6.2, for a pH shift of .8, the exact same as in case 1.

One possible explanation for this myth is that many copies of this pH chart skip some of the higher pH values, for example, jumping from pH 7.4 to a pH of 8.0. If the reader didn't pay careful attention, they might mis-interpret the size of the pH swing. I specifically made sure to include all pH values, between 6 and 8, in steps of .2.

This relationship will break down at extremely low KH levels (below 1 degree), when there isn't enough carbonate to completely buffer the acids present. In that case, the pH can drop quickly and dramatically. But if the KH is 1 degree or higher, then the size of the pH swing when injecting CO2 will be determined only by the amount of CO2 dissolved in the water.

The pH-KH-CO2 Relationship
pH, KH, and CO2 have a fixed relationship as long as carbonate is the only buffer present (no phosphate buffers like pH-UP and- DOWN, Discus Buffer, etc). There are some parts of the country that have high levels of phosphates in their water supply. For those cases, determining CO2 levels will be difficult, as the phosphate will throw off the pH-KH-CO2 relationship, which means the CO2 charts and calculator below won't work. Note that the commercially available CO2 test kits will also be invalidated by the phosphates.

To determine your CO2 level based on the pH and KH, you can enter the values into the on-line calculator, or use the chart at the bottom of the page.

NOTE: If you aren't adding CO2 to your water, and the CO2 level based on the pH and KH indicates more than 5ppm, then it is very likely that some other buffer (such as phosphate) is present in your water. In an inhabited aquarium, the amount of CO2 produced by the fish will not have an effect on CO2 levels in the water. Any excess CO2 created by fish will dissipate into the air, leaving a fairly constant CO2 level of about 3-4ppm. If you test your pH and KH, and without adding any CO2, the chart says you've got 20ppm CO2, don't believe it.

In some case, water coming right from the tap can contain very high or very low levels of CO2. This can result in tap water with a high KH, and low pH. But, in just a few hours, that excess CO2 will dissipate from the water, leaving the normal 3-4ppm, and the pH will rise. Sometimes, the water might come from the tap with extremely little CO2, which can result in tap water with a low KH, and a very high pH. Again, after a few hours, the CO2 level will equalize, and the water will end up with 3-4ppm CO2.

CO2/pH/KH calculator and chart
NOTE: This calculator (and the chart based on this formula) will only work if your water is carbonate buffered. If your water contains high levels of phosphates, it will alter your water properties, and invalidate these CO2 calculations.
If you have measured your pH and KH, and want to know how much CO2 you have, enter the pH and KH here.

An alternate use for this calculator, or the chart below, would be to determine the "target" pH needed to achieve a certain amount of CO2. A desireable CO2 level is 10-25ppm (which is indicated in green on the chart). Levels below that don't provide optimum CO2 concentrations for high plant growth. CO2 concentrations over 25ppm can be harmful to the inhabitants of your tank. I typically shoot for 15ppm CO2. So, using the calculator, enter your KH, then try entering different pH values until it shows a CO2 level around 15ppm. Using the chart, just find the row that contains your KH, then go across until you find your desired CO2 level, then look to the top of the column to see what your "target" pH should be. Once you know that, you can adjust your CO2 injection to hit that target pH.


Myth: CO2 level can be adjusted simply by adding chemicals to alter the KH or pH.
This is a common misconception when using the CO2 / KH / pH table. It appears that by altering any parameter, the other values should move. But this is not true. Treat the pH value you see as a result. If you alter the KH, then the pH will move. If you alter the CO2 level, then the pH will move. The pH will always react to changes in either of the other two parameters.

Example: My water comes out of the tap with a KH of 3 degrees, and a pH of 7.6, which according to the the indicates a CO2 level of 2.3ppm. Looking at the chart, I might (incorrectly) assume that If I simply raised my KH to 10 degrees, I would end up with the same pH, but the CO2 level would now be 12ppm! How easy! I can add CO2 just by adding some baking soda to raise my KH.
BUT! it doesn't work that way. Instead, as I raise the KH, the pH will rise along with it, and the indicated CO2 level staying at it's 2.3ppm. In my case, if I raised the KH to 10 degrees,

You can not alter the KH levels other than by adding or removing carbonate. You can not alter the CO2 levels other than by adding or removing CO2.

Adding certain "pH altering additives" can cause much confusion as well. Additives like "Proper pH 7.0" which force the pH to a certain value completely invalidate the CO2 / KH / pH relationship. This is because these pH altering additives contain phosphates. Phosphates replace the carbonates in the buffering system. And the CO2 / KH / pH relationship is only valid in a system that is buffered by Carbonates.

There is on case I've seen where the addition of CO2 resulted in an increase in KH. This can happen when you have something in the tank that dissolves carbonate into the water. Seashells, crushed coral, and many gravels and rocks will do this. With the addition of CO2, the water turns more acidic, which will increase the dissolving of the minerals. It appears that increasing CO2 raises the KH, which isn't really the case. The dissolving minerals raise the KH, and the increase in KH results in an increase in pH. In a system using a pH probe and controller to regulate CO2 levels, this can have fatal consequences, since the pH controller will keep trying to lower the pH, but as more CO2 is dissolved, it lowers the pH, which raises the KH, which raises the pH. So you now have more CO2, but the same pH. So the controller adds even MORE co2. And it will keep going. So it's important to know your KH whenever using pH to judge CO2 levels.


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## Ashattuck6400 (Feb 23, 2014)

*Additional question*

There is some great information here! I have an additional question. My ph just before my lights and co2 come on is 7.6. My ph just before my lights and co2 go off is 6.8. MY KH is 4. I currently do not have fish in my tank, but hope to get them soon. My question is, is it OK that my ph is fluctuating this much every day???


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## monster mac (Mar 15, 2014)

Ashattuck6400 said:


> There is some great information here! I have an additional question. My ph just before my lights and co2 come on is 7.6. My ph just before my lights and co2 go off is 6.8. MY KH is 4. I currently do not have fish in my tank, but hope to get them soon. My question is, is it OK that my ph is fluctuating this much every day???


Was looking for this exact same Q and A. Hope someone chimes in.


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## dinnese (Feb 4, 2013)

You see a drop in ph because co2 forms carbonic acid with water, your actual ph never changes. If co2 actually changed the physical make up of water (GH and KH) then there would be no use for RO water systems. Co2 does not remove GH or KH, these are the factors that alter the real ph of your water. 
This ph drop is a false ph drop. Think of it as co2 tricking your ph test kits or ph controller. Test kits and controllers are simply responding to contact of the co2 acid, the physical nature of the the water never changes, always remains the same. You can inject co2 until the ph controller reads 5.0 if you wanted to, the real ph is still the same. We know this because the minute you take a sample of the water in a cup, shake it up to gas off the co2 the ph go's right back up to where you started. Never once did you remove GH or KH from the water, the only real way to ever alter the ph is to use RO water, or rain water. These types of water contain no GH or KH, no minerals.


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## jrman83 (Nov 22, 2010)

My ph swings a full point from CO2 and have never seen any affect on my fish. As long as your kh is in the 3-5 range you should be okay.

Sent with my Samsung S4 via Tapatalk


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## Diana (Jan 14, 2010)

Fish can handle the pH change that happens with CO2 just fine. The mineral levels are stable, and that is more important to the fish.


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## monster mac (Mar 15, 2014)

dinnese said:


> You see a drop in ph because co2 forms carbonic acid with water, your actual ph never changes. If co2 actually changed the physical make up of water (GH and KH) then there would be no use for RO water systems. Co2 does not remove GH or KH, these are the factors that alter the real ph of your water.
> This ph drop is a false ph drop. Think of it as co2 tricking your ph test kits or ph controller. Test kits and controllers are simply responding to contact of the co2 acid, the physical nature of the the water never changes, always remains the same. You can inject co2 until the ph controller reads 5.0 if you wanted to, the real ph is still the same. We know this because the minute you take a sample of the water in a cup, shake it up to gas off the co2 the ph go's right back up to where you started. Never once did you remove GH or KH from the water, the only real way to ever alter the ph is to use RO water, or rain water. These types of water contain no GH or KH, no minerals.


Outstanding, thank for the break down. I feel much more comfortable with the CO2 PH swings. Thanks everyone


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## Hoppy (Dec 24, 2005)

dinnese said:


> You see a drop in ph because co2 forms carbonic acid with water, your actual ph never changes. If co2 actually changed the physical make up of water (GH and KH) then there would be no use for RO water systems. Co2 does not remove GH or KH, these are the factors that alter the real ph of your water.
> This ph drop is a false ph drop. Think of it as co2 tricking your ph test kits or ph controller. Test kits and controllers are simply responding to contact of the co2 acid, the physical nature of the the water never changes, always remains the same. You can inject co2 until the ph controller reads 5.0 if you wanted to, the real ph is still the same. We know this because the minute you take a sample of the water in a cup, shake it up to gas off the co2 the ph go's right back up to where you started. Never once did you remove GH or KH from the water, the only real way to ever alter the ph is to use RO water, or rain water. These types of water contain no GH or KH, no minerals.


I don't agree. The pH change is real. When anything in the water changes the acidity of the water the pH is a measure of that. Adding an acid, or something that is converted to an acid to any degree, as CO2 is, changes the surplus of H+ ions in the water, which is what a change in pH is. When you take a sample of the water in the tank and shake it up, you are removing the CO2, which also causes the amount of carbonic acid in the water to drop, reducing the surplus of H+ ions, as indicated by the rise in pH. Aquarium water pH is a result of several things in the water, it is a consequence of "stuff" being in the water. What harms fish is some of that "stuff", not the pH itself. (This is advanced chemistry explained by someone with only bare knowledge of any chemistry.):icon_cool


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## Ashattuck6400 (Feb 23, 2014)

Thank you for the responses! All responses seem legit. Would the following conclusion be correct....the ph swings are real,but at long as all the 'stuff' (aside from co2) in the water is stable my fish will be Ok?? Please correct if I am completely wrong.


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## fablau (Feb 7, 2009)

Ok, explain me why I have never been able to read the KH/PH chart: I have KH 7 and the PH from tap is 7.4. According to the chart, I'd need to lower my PH to just 6.9 to approach 30ppm of co2... Correct?

But the truth is, with my 75gl tank, EI and a big wet/dry filter, I need to lower the PH to 6.1 to see my plants grow enough well with very little algae. Fish are good, and yet the chart would give me a crazy 166ppm co2 concentration!

That cannot be the case, right? Please, note I don't use any PH buffer nor a tap water softener... And my tap water doesn't contain phosphates...

So, what's wrong?


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## fablau (Feb 7, 2009)

Ashattuck6400 said:


> Thank you for the responses! All responses seem legit. Would the following conclusion be correct....the ph swings are real,but at long as all the 'stuff' (aside from co2) in the water is stable my fish will be Ok?? Please correct if I am completely wrong.


Yes, that's correct.


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## Hoppy (Dec 24, 2005)

fablau said:


> Ok, explain me why I have never been able to read the KH/PH chart: I have KH 7 and the PH from tap is 7.4. According to the chart, I'd need to lower my PH to just 6.9 to approach 30ppm of co2... Correct?
> 
> But the truth is, with my 75gl tank, EI and a big wet/dry filter, I need to lower the PH to 6.1 to see my plants grow enough well with very little algae. Fish are good, and yet the chart would give me a crazy 166ppm co2 concentration!
> 
> ...


Phosphates aren't the only non-CO2 thing in water that affects the pH. We have organic acids from wood or decaying leaves that also lower the pH. The relationship between carbonate hardness, CO2 concentration and pH is only applicable for pure, distilled water with nothing else dissolved in it except a carbonate compound and CO2. Add some tannic acid and the relationship breaks down. If this were not the case you could gain CO2 by adding hydrochloric acid to the water, thus lowering the pH. But, you only gain CO2 in the water by dissolving CO2 in the water.


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## Hoppy (Dec 24, 2005)

fablau said:


> Yes, that's correct.


I agree!:biggrin:


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## fablau (Feb 7, 2009)

Hoppy said:


> Phosphates aren't the only non-CO2 thing in water that affects the pH. We have organic acids from wood or decaying leaves that also lower the pH. The relationship between carbonate hardness, CO2 concentration and pH is only applicable for pure, distilled water with nothing else dissolved in it except a carbonate compound and CO2. Add some tannic acid and the relationship breaks down. If this were not the case you could gain CO2 by adding hydrochloric acid to the water, thus lowering the pH. But, you only gain CO2 in the water by dissolving CO2 in the water.


Well, I appreciate your answer, but doesn't explain my question... I don't think to be the only one experiencing this mismatch. For me the chart is completely useless.


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## Hoppy (Dec 24, 2005)

fablau said:


> Well, I appreciate your answer, but doesn't explain my question... I don't think to be the only one experiencing this mismatch. For me the chart is completely useless.


The chart is useless for determining how much CO2 is in the aquarium water. It does have value for when you are working with distilled water and baking soda, doing experiments. Measuring how much CO2 is in the aquarium water is very difficult unless you have a few thousand dollars to spare for a quality CO2 measuring probe. You can assume you have a reasonably high CO2 concentration if you get a 1 pH drop when you turn on your CO2 after allowing lots of time for the CO2 that was in the tank to dissipate. It will probably be around 30 ppm, but it could be much lower or higher.

It is better to judge whether you have an optimum amount of CO2 in the water by observing the plants and fish, and making very small changes in CO2 bubble rate until both the plants and the fish are doing well.


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## fablau (Feb 7, 2009)

This is a very nice answer, and pretty much my own thinking! Makes perfectly sense, thank you!

So, I guess, if you use tap water, you have just one option: experiment until you find the right amount of co2 your plants need, and the so called "sweet spot". So hard to find for me!


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## dzega (Apr 22, 2013)

fablau said:


> Ok, explain me why I have never been able to read the KH/PH chart: I have KH 7 and the PH from tap is 7.4. According to the chart, I'd need to lower my PH to just 6.9 to approach 30ppm of co2... Correct?
> 
> But the truth is, with my 75gl tank, EI and a big wet/dry filter, I need to lower the PH to 6.1 to see my plants grow enough well with very little algae. Fish are good, and yet the chart would give me a crazy 166ppm co2 concentration!
> 
> ...


same here. my Kh>7
with CO2 PH drops to <6.6 and reading chart that gives me loads of co2 wich cant be true using diy yeast system


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## Hoppy (Dec 24, 2005)

dzega said:


> same here. my Kh>7
> with CO2 PH drops to <6.6 and reading chart that gives me loads of co2 wich cant be true using diy yeast system


What is the starting pH when it drops to 6.6? Remember, the drop in pH with CO2 addition does tell you the ratio of the final ppm of CO2 to the initial ppm of CO2.


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## momon6 (Mar 23, 2015)

Hi guys, please do help me out here.

my ph is at 6.8 with a kh of less than 1? i don't understand.???

And when the co2 is turned on my ph is at 5.8. its too much for my plants. 


will i just add baking soda? to solve my problem. about 1 teaspoon will increase my kh about 4 for a 50liter tank.


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## imcmaster (Jan 30, 2015)

It is likely that your water supply has very little buffering/kh so your tank kH is very low. To have a pH of 6.8 without CO2 implies your aquarium is slightly acidic ... do you use aquasoil or peat, etc which has the effect of lowering the pH of the water?

It is generally accepted that if you are using CO2 that you should maintain a kH value of 3 or higher, so that the pH high/low marks are more predictable. There is a danger of pH dropping too low otherwise (if kH is very low).

You should be encouraged to use the baking soda to increase kH. Do so in a careful manner; adding kH will cause pH to rise, and you do not want to raise your pH quickly.


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## momon6 (Mar 23, 2015)

imcmaster said:


> It is likely that your water supply has very little buffering/kh so your tank kH is very low. To have a pH of 6.8 without CO2 implies your aquarium is slightly acidic ... do you use aquasoil or peat, etc which has the effect of lowering the pH of the water?
> 
> It is generally accepted that if you are using CO2 that you should maintain a kH value of 3 or higher, so that the pH high/low marks are more predictable. There is a danger of pH dropping too low otherwise (if kH is very low).
> 
> You should be encouraged to use the baking soda to increase kH. Do so in a careful manner; adding kH will cause pH to rise, and you do not want to raise your pH quickly.


Thanks! just did add baking soda to my tank. hopefully it will change everything.

just using amazonia ADA soil with no peat soil at oil in the filter. just the ordinary stock in an Eheim filter.


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## momon6 (Mar 23, 2015)

guys, do i need to those baking soda everytime because just the other day i dose it then i got a very good result with a kh of 4 degrees but apparently when i tested it again its back to 1 degree. help guys...


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## imcmaster (Jan 30, 2015)

Have you done a water change during this time? If so, how much?


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## momon6 (Mar 23, 2015)

imcmaster said:


> Have you done a water change during this time? If so, how much?


nope i have done any water change yet.like on wednesday i add baking soda and this time when i check the kh it turned back to 1 degree. no water change was done.


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## imcmaster (Jan 30, 2015)

It is hard to say what is going on. You have acidic properties (soil, etc) that are driving the pH down (and kH will follow down). Perhaps you can adapt to a lower pH and manage. What is your principal reason for wanting a higher pH (what symptom of the lower pH do you observe).
In theory you should be able to stabilize a higher pH using an alkaline buffer, but for you it seems you will be fighting to do this. It is up to you - you could keep trying to obtain higher kH to reach a better pH balance, or you could try to manage at the lower pH.


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## momon6 (Mar 23, 2015)

i was thinking maybe this was the reason why my glosso plant are dying since the water is just too acidic. but they say i lacked light source. i just only have 30 LED watts for my 10 gallon tank.

heres the pic,








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## imcmaster (Jan 30, 2015)

Yes, I agree, this plant likes high light. This plant should be fine at the pH of your tank (if all the other factors including light, CO2 are appropriate).


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## Plecosaurus Rex (Jan 8, 2017)

Hoppy said:


> I don't agree. The pH change is real. When anything in the water changes the acidity of the water the pH is a measure of that. Adding an acid, or something that is converted to an acid to any degree, as CO2 is, changes the surplus of H+ ions in the water, which is what a change in pH is. When you take a sample of the water in the tank and shake it up, you are removing the CO2, which also causes the amount of carbonic acid in the water to drop, reducing the surplus of H+ ions, as indicated by the rise in pH. Aquarium water pH is a result of several things in the water, it is a consequence of "stuff" being in the water. What harms fish is some of that "stuff", not the pH itself. (This is advanced chemistry explained by someone with only bare knowledge of any chemistry.):icon_cool


I would agree that CO2 injection does alter the _actual_ pH. CO2 reacts with water to form carbonic acid. CO2 + H20 -> H2CO3. H2CO3 dissociates in water into H(+)(aqueous) and HCO3(-)(aqueous). The H+ will raise the actual Ph, as the H+ concentration in water is the very definition of pH. However, This is a reversible reaction, and will eventually (even quickly) reverse. H(+) + HCO3 -> CO2(gas) + H2O. As for the effect on fish, I can't speak with any real authority.

I'm a non-practicing, but degreed chemist.


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