Are CO2 levels important?

[Aqua Man]

@arking_mark what will your target alk be now? Ive been in this rabbit hole for a bit also. Great discussion!

Alkalinity is the capacity of water to resist acidification.

If a solution has a low ph it’s acidic and high ph is considered basic. So it seems that a higher Alk would be preferred to combat acidification, low ph.

the pH is solely controlled by the concentration of carbon dioxide in the atmosphere above the system.

I think I’m getting there. What I’m missing is why doesn’t my buffer of choice counteract the infusion of CO2?
That is if the definition of Alkalinity above is correct.

 

[arking_mark]

@arking_mark what will your target alk be now? Ive been in this rabbit hole for a bit also. Great discussion!

Alkalinity is the capacity of water to resist acidification.

If a solution has a low ph it’s acidic and high ph is considered basic. So it seems that a higher Alk would be preferred to combat acidification, low ph.




I think I’m getting there. What I’m missing is why doesn’t my buffer of choice counteract the infusion of CO2?
That is if the definition of Alkalinity above is correct.

Maybe the best way to think about this is with an example. Let's assume I have my tank roughly stable at pH 8.3 and Alk at 7.7dKH. This would mean that my effective pCO2 is roughly at 350.

I decide I want to up my Alk to 9dKH and dump in 1.3dKH worth of baking soda (sodium bicarbonate). This would impact the speciation of the bicarbonate and carbonate and effectively lower my pH (increase CO2 level).

Over time, that my effective pCO2 would re-equalize the CO2 levels within the tank and result in roughly an 8.35 pH

@Randy Holmes-Farley @Dkeller_nc Did I get this right?

 

[arking_mark]

Maybe the best way to think about this is with an example. Let's assume I have my tank roughly stable at pH 8.3 and Alk at 7.7dKH. This would mean that my effective pCO2 is roughly at 350.

I decide I want to up my Alk to 9dKH and dump in 1.3dKH worth of baking soda (sodium bicarbonate). This would impact the speciation of the bicarbonate and carbonate and effectively lower my pH (increase CO2 level).

Over time, that my effective pCO2 would re-equalize the CO2 levels within the tank and result in roughly an 8.2pH

@Randy Holmes-Farley @Dkeller_nc Did I get this right?

If I got this right, in my situation, the pH-controlled scrubber would come on less and not remove as much CO2 to maintain my desired 8.3 pH. My equivalent pCO2 would be roughly 400.

 

[Randy Holmes-Farley]

So I think I have a light bulb moment here!

With the understanding that pCO2 levels just influence the pH and carbonate speciation relationship I can better understand both cases.

For bath case 1 and case 2, the pCO2 will determine how pH and Alk behave with each other. If I raise Alk, pH will go up and visa-versa. If I increase the pCO2, Alk remains the same, but pH will drop. (By the way, for both case 1 and case 2, the pCO2 is the calculated relationship with the pH and Alk, meaning the system is stable.)

So which system is preferred? It would seem both cases provide adequate Alk and pH levels to maintain coral. However, case 2 (elevated pCO2) may be preferred due to the ability to maintain stable elevated Alk at a desired pH as some anecdotal evidence and some studies have shown increased coral growth. However, at this level, abiotic consumption is also increased and possibly a significant amount of CaCO3 may just be precipitated out. Also at higher Alk, in ULN systems, coral may experience issues with "burnt tips".

Higher alk and higher pH both tend to drive faster growth of some hard corals.

Is either better than the other? Don't know that anyone has ever really compared.

Is either "better" than slower growth at lower levels of both? Depends on what you want to accomplish.

This thread has a more detailed discussion on alk vs ph stability and such:

Alkalinity stability? pH stability? Are they even different?

It is currently the dogma of the reef aquarium hobby to say that alkalinity stability is very important for SPS corals, and that pH is not. Often, the very idea of someone trying to optimize pH is called “chasing numbers” as if that statement by itself shows it to be foolish. Since I have...

www.reef2reef.com

 

[Randy Holmes-Farley]

Maybe the best way to think about this is with an example. Let's assume I have my tank roughly stable at pH 8.3 and Alk at 7.7dKH. This would mean that my effective pCO2 is roughly at 350.

I decide I want to up my Alk to 9dKH and dump in 1.3dKH worth of baking soda (sodium bicarbonate). This would impact the speciation of the bicarbonate and carbonate and effectively lower my pH (increase CO2 level).

Over time, that my effective pCO2 would re-equalize the CO2 levels within the tank and result in roughly an 8.35 pH

@Randy Holmes-Farley @Dkeller_nc Did I get this right?

Yes. Tanks are usually not at equilibrium with the air though, so pH is always moving around even if alk is stable.

 

[Dkeller_nc]

To add to Randy's note about both high pH and higher alkalinity (possibly) accelerating coral growth - this is actually what I was trying to explain in one of the first posts on this thread. It is possible (though not definitive) that the effect of higher pH and higher alkalinity on coral growth (i.e., accelerating it) is just two sides of the same coin.

Specifically, if one surmises that coral growth is related to the availability of the carbonate ion (CO3^-2), a higher pH would mean that the equilibrium of carbonic acid/bicarbonate ion/carbonate ion would shifted to the form of the carbonate ion (instead of bicarbonate ion and/or carbonic acid, free CO2). That means that the molar concentration of the carbonate ion in the water is higher at a higher pH, even though the total alkalinity is precisely the same.

Alternatively, it's possible to have a higher molar concentration of the carbonate ion if the total alkalinity in the system is increased, even though the pH remains the same.

One reefer's caution (as opposed to a theoretical chemistry/biology discussion) about messing with one's water chemistry in this regard. If you have substantial quantities of sensitive SPS such as acropora in your tank, be extremely cautious about making substantial changes to your water's average alkalinity. There are two reasons for this - one is that sensitive SPS species don't tolerate rapid changes in alkalinity, and the second is what Randy posted earlier in the thread - if you're running an overall higher alkalinity, you are very likely to have to run higher levels of dissolved inorganic nutrients in the form of nitrates and phosphates. Those that have run ultra low nutrient systems (ULNS) whether intentionally or otherwise over the last 10 to 15 years have experienced coral loss of color and/or outright bleaching from the combination of high alkalinity (>8 dKH) and low nutrients (< 5 ppm NO3, <100 ppb PO4).

If you choose to experiment with your system and you have SPS, particularly acropora species, I wouldn't change the average alkalinity by more than 1 dKH unit per week, and I wouldn't deviate more than 2 dKH from the typical 8 dKH that most folks keep SPS tanks at. And while I was evaluating these changes, I'd keep a close eye on dissolved inorganic nitrate and phosphate and on the appearance of your SPS. If their colors start to fade, I'd increase your feeding of the fish and corals in your tank, and slowly allow the alkalinity to fall by reduction in 2-part dosing (or backing off on the output of a calcium reactor if that's your chosen method).

 

[arking_mark]

To add to Randy's note about both high pH and higher alkalinity (possibly) accelerating coral growth - this is actually what I was trying to explain in one of the first posts on this thread. It is possible (though not definitive) that the effect of higher pH and higher alkalinity on coral growth (i.e., accelerating it) is just two sides of the same coin.

Specifically, if one surmises that coral growth is related to the availability of the carbonate ion (CO3^-2), a higher pH would mean that the equilibrium of carbonic acid/bicarbonate ion/carbonate ion would shifted to the form of the carbonate ion (instead of bicarbonate ion and/or carbonic acid, free CO2). That means that the molar concentration of the carbonate ion in the water is higher at a higher pH, even though the total alkalinity is precisely the same.

Alternatively, it's possible to have a higher molar concentration of the carbonate ion if the total alkalinity in the system is increased, even though the pH remains the same.

One reefer's caution (as opposed to a theoretical chemistry/biology discussion) about messing with one's water chemistry in this regard. If you have substantial quantities of sensitive SPS such as acropora in your tank, be extremely cautious about making substantial changes to your water's average alkalinity. There are two reasons for this - one is that sensitive SPS species don't tolerate rapid changes in alkalinity, and the second is what Randy posted earlier in the thread - if you're running an overall higher alkalinity, you are very likely to have to run higher levels of dissolved inorganic nutrients in the form of nitrates and phosphates. Those that have run ultra low nutrient systems (ULNS) whether intentionally or otherwise over the last 10 to 15 years have experienced coral loss of color and/or outright bleaching from the combination of high alkalinity (>8 dKH) and low nutrients (< 5 ppm NO3, <100 ppb PO4).

If you choose to experiment with your system and you have SPS, particularly acropora species, I wouldn't change the average alkalinity by more than 1 dKH unit per week, and I wouldn't deviate more than 2 dKH from the typical 8 dKH that most folks keep SPS tanks at. And while I was evaluating these changes, I'd keep a close eye on dissolved inorganic nitrate and phosphate and on the appearance of your SPS. If their colors start to fade, I'd increase your feeding of the fish and corals in your tank, and slowly allow the alkalinity to fall by reduction in 2-part dosing (or backing off on the output of a calcium reactor if that's your chosen method).

So unintentionally, I'm running ULN and hence targeting my dKH below 8. I'm trying to get to heavy in/out. I have yet to be successful with corals in this tank. I kinda figured I'd be there by now (6 months), but maybe turning the corner soon. I'm finally seeing coralline algae and some frags are starting to survive.