# Overnight pH changes with CO2 theory



## jameson_uk (26 Nov 2016)

I have been considering adding CO2 for a while but one thing that has been bugging me is the fluctuating pH when CO2 is not being added.

I just wanted to understand the theory as to what happens overnight.  During the day (when the lights are on) I guess you are constantly adding CO2 but this is also constantly being consumed by plants (and to some extend degassed) so I get that with some playing you get figure out the amount of CO2 to add to keep the level steady.   

What I don't get it what happens when you turn off the lights and stop adding CO2.  I guess that the plants stop consuming CO2 (in fact they start respiring) but the degassing will continue (and I have read that you should probably run an airstone overnight to actually increase the degassing).   My KH5 / pH 7.4 tank should have 6ppm CO2 and I guess there will be some variation overnight already but this is going to be small scale as there is only a small amount to start with.   If I was to alter the pH to about 6.8/6.9 I would be having to have CO2 of 19-24ppm but if I then stopped adding CO2, turned the lights off and turned on an airstone the CO2 level will drop but will it drop to pre-injected levels?   If so this is a swing of pH 6.8 to 7.4 over a few hours and I believe when changing the pH for fish you should be doing something like 0.1 change per day ?????


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## zozo (26 Nov 2016)

Thinking about it, this is something to take into consideration if you run the system on the edge at very high co2 levels.. 30ppm is just an average goal, but many people go far beyond that and go with a almost yellow drop checker to get ultimate results. Since the kh/ph chart used is just a bad approximate.. I've tested it myself with a simular kH as yours and kept adding till the turning point of almost yellow dropchecker and measured a pH 6.2 which is according the chart 130 ppm co2 with a dkH6.. You can build very slowly toward this and the fish addapt, probably acclimatize to it the same as we do when we go high up into the mountains. With running such a regime you also need to very carefull with adding new tank mates, fish not slowly acclimatized to this will sufecate immediately.. 

But then if you stop adding and the lights go out, plants turn their photosythesis around and start adding co2 as well instead of oxygene. So if you are already at a dangerous turning point chances are high the fish will suffer and or maybe not survive the night. So adding an airstone for extra agitation to force the co2 degassing over night might be necessary. 

Some people with very difficult plants run their tanks like this to maximize plant health... I have no idea if the fish suffer more from oxygen depriviation or from Ph swing like this, all tho they look like not realy being affected. Hence how would one know as long as they do not gasp for air and behave normal. Swimming around like nothing is wrong is all they can do, how it affects them on the long term is a best guess. 

In my own personal opinion i find it an absolut absurdity to house fish in such a regime. But that said, it is strickly peronal and i worry about my fish a lot and there for go very lean on co2. But can not judge people who think differntly.. Since i also do not grow difficult plants i see no need to go that high on co2 and keep it around the pH 6.8 - 6.9 which is also around the 20ppm it does the job for me more than enough.. And the tank is after 18 months so well matured i'm even considering converting it back to low tech again and save the co2 maybe for later projects..

Like this my pH swing is also only about 0.5 over night..


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## zozo (26 Nov 2016)

Oh might add, that's about the same pH swing i got in the low tech tank which is natural due the plants photosynthesis.


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## dw1305 (26 Nov 2016)

Hi all,





jameson_uk said:


> My KH5 / pH 7.4 tank should have 6ppm CO2


You won't really have 6ppm CO2, the theoretical value for dissolved CO2 (at equilibrium with atmospheric CO2) is about 0.5ppm, although this is usually assumed to be higher, partially because CO2 levels in an average room will be higher than 400ppm CO2 in the atmosphere. My suspicion would be that the normal CO2 level lies somewhere in between 0.5 ppm and the 3ppm that is often assumed.  





jameson_uk said:


> If I was to alter the pH to about 6.8/6.9 I would be having to have CO2 of 19-24ppm but if I then stopped adding CO2, turned the lights off and turned on an airstone the CO2 level will drop but will it drop to pre-injected levels? If so this is a swing of pH 6.8 to 7.4 over a few hours and I believe when changing the pH for fish you should be doing something like 0.1 change per day ?????


Hard water, like Lake Tanganyika or the sea, is almost infinitely carbonate buffered and you need huge changes in water chemistry to change the pH. In pure H2O pH is a basically meaningless and any addition of an acid, or base, will cause huge changes in pH. Many people who comment on pH on forums etc. don't fully understand this.

When you add CO2 to your tank water you change the carbonate ~ bicarbonate ~ CO2 equilibrium in favour of CO2 and more H2CO3 goes into solution, as H+ and HCO3-. Acids are defined as H+ ion donors and the pH falls. When the water degasses the pH will return to it's starting value as CO2 levels fall, and CO3 comes out of solution (usually as CaCO3, the least soluble carbonate).

cheers Darrel


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## sciencefiction (27 Nov 2016)

zozo said:


> Some people with very difficult plants run their tanks like this to maximize plant health... I have no idea if the fish suffer more from oxygen depriviation or from Ph swing like this, all tho they look like not realy being affected.





jameson_uk said:


> If so this is a swing of pH 6.8 to 7.4 over a few hours and I believe when changing the pH for fish you should be doing something like 0.1 change per day ?????



CO2 only has an impact on the Ph and not the Kh. It is the Kh swing that fish are sensitive to in aquariums. Hence, you should not worry about a Ph swing caused by CO2 fluctuations. Theoretically, there is an extremely minimal Kh impact from CO2 injection but not near enough to affect fish in any way.


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## zozo (27 Nov 2016)

I also did read some articles about changing (lowering) pH in fishtanks where authors state you should gradualy lower it with 0.1 unit per day when adding chemical (extracts) or even with peat. But as Darrel already sais, goes for me the same, i do not realy fully understand the whole pH versus kH cycle and how it realy rellates to eachother. Have to read the whole story 10 times more and try to get my head around it and try not to forget it again. Peat for example changes kH wich reflects also on the pH. It obviously absorbs more carbonate than it adds acids i guess..

Also have no idea how they come to the conclusion of fish beeing so sensitive to pH swings in general. In nature it happens reportedly rather high in some regions during one day measured several units difference fluctuating from morning to evening. And as said if you monitor a well planted and well lit low tech tank the pH also fluctuates much more within 24 hours than the recomended 0.1 unit.

Tho Ph stability reportedly seems to be important for breeding some community fish species.. But that is again a different area then keeping. 

It's more to worry about the co2 and think about that, if you realy need so much as what is regularly addviced, than about the pH.. That was what i was trying to say...


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## sciencefiction (27 Nov 2016)

zozo said:


> But as Darrel already sais, goes for me the same, i do not realy fully understand the whole pH versus kH cycle and how it realy rellates to eachother.



Explained below by a member on another forum. It's an old thread and not scientific paper but that's how it roughly works in simple maths...It's easier for me to understand it that way...so maybe it will help some similar minds...

_1) I understand that most of our dissolved CO2 is CO2(aq), not H2CO3.
2) Carbonic acid is a weak acid and only partially dissociates.
3) The partial dissociation of the H2CO3 will have a much greater effect on pH than on KH, due to the scale of the concentrations. ie, It doesn't take much H+ from the H2CO3 to change the pH, but it takes much more HCO3- to affect the KH.

Doing the math (finally):

ph 7 = 1x10^-7 M H+
KH 10 = 107.1ppm CO3-- = 1.78x10^-3 M CO3--

*Already we see a difference of x10^4. It would take 1000 times more HCO3- than H+ to get an equivilent change in KH.*
_
source link: _http://www.aquaticplantcentral.com/...10902-why-doesnt-co2-injection-affect-kh.html_


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