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I am an aquarist, and I am looking into adding CO2 into my aquarium in order to help plant growth.

This is a pretty standard practice, but some of the science spread within the community is contradictory.

My main question is: will adding surface agitation, or aeration to an aquarium with high levels of CO2, cause the CO2 to dissipate from the water?

My understanding is that, surface agitation with cause the water to ‘equalize’. Meaning the fluid with less CO2 (ambient air or water), will ‘steal’ CO2 from the liquid that has more until they are equal.

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At equilibrium, the concentration of CO2 in the water will not equal the concentration of CO2 in the air. Instead, at equilibrium, their chemical potentials will be equal. At room temperature, CO2 likes being in a gaseous state more than in a aqueous state. Consequently, when their chemical potentials are equal, the concentration of CO2 in the water will be quite a bit less than that in the air.

Specifically:

According to https://climate.nasa.gov/news/2915/the-atmosphere-getting-a-handle-on-carbon-dioxide/ , the current concentration of CO2 in the air is 412 ppm. Extrapolating from this table (https://sites.chem.colostate.edu/diverdi/all_courses/CRC%20reference%20data/solubility%20of%20carbon%20dioxide%20in%20water.pdf) yields the following equilibrium concentrations of CO2 in water:

At 20 C: 29 ppm

At 25 C: 26 ppm

Extensive aeration, with room air, will help to maintain these equilibrium concentrations (you might want to consider a bubble wand in addition to air stones). Surface agitation would also help, but I would think the increase in surface area you would get with extensive aeration would be greater than that you would get with surface agitation. Plus, with surface agitation, you might end up with a concentration gradient of CO2. By contrast, aeration will help ensure the aquarium is equilibrated with the air at all depths.

Note also that the 412 ppm value is for outdoor air. Indoor air can be twice that or more, especially in a poorly-ventilated room with people. The concentration of CO2 in water is approximately linearly proportional to its concentration in air, so if the indoor air were ~800 ppm, then the corresponding equilibrium concentrations in the water would be about:

At 20 C: 60 ppm

At 25 C: 50 ppm

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  • $\begingroup$ This is awesome! Thanks for all the information. Just so I can make sure I understand. At room temperature, 25C agitated water will absorb CO2 until it’s reached 26ppm (assuming the CO2 levels are the same as outside). If I added CO2 and brought the level up to 30ppm, the CO2 would drop to 26ppm? $\endgroup$
    – JarredAwesome
    Commented Jan 20, 2020 at 13:39
  • $\begingroup$ @JarredAwesome That's right. If you bubble in CO2 whose partial pressure is greater than that of the atmospheric CO2 (easily achievable with, say, pure CO2, since there its partial pressure will be ~ 1 atm), you will increase the concentration of CO2 in the water. If you stop bubbling in the CO2, it will drop back to the equilibrium concentration (in this case, 26 ppm). [Of course, you don't truly have an equil. sys, since the plants&fish perturb the conc of CO2; heavy aeration w/ ambient air should mostly overcome this, giving a steady-state conc. of CO2 about equal to the equil. value.] $\endgroup$
    – theorist
    Commented Jan 20, 2020 at 22:05
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Some gases can displace dissolved air by a simple process of bubbling. The formal wording for it is "sparging" For example, it was a standard practice to bubble helium in organic solvents to remove dissolve air. Similarly, if you sparge $\ce{N2}$ for a long time in water, you will displace oxygen and dissolved air from water. I feel that exposing water to high levels of carbon dioxide may not be a good idea for the fishes because you will deplete dissolved oxygen.

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  • $\begingroup$ The co2 is added in very low amounts. There is equipment we use to make sure we don’t add too much. So in this case, would surface agitation remove dissolved co2 from the water? $\endgroup$
    – JarredAwesome
    Commented Jan 19, 2020 at 17:45
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    $\begingroup$ I don't think so because water has an affinity for carbon dioxide (it is not completely inert like oxygen, nitrogen etc towards water). But let us go back to nature. Search the amount dissolved carbon dioxide in seawater and in rivers and see what are the levels in air. They are highly agitated systems. $\endgroup$
    – ACR
    Commented Jan 19, 2020 at 17:48
  • $\begingroup$ I just took a look. I’m not a chemist, so my understanding is pretty base level. According to to the attache link, co2 in rivers wants to reach equilibrium but doesn’t due to other factors. Ideal ppm in an aquarium is 30ppm co2. And the co2 indoors is more then 10 times that. So I would assume the surface agitation adds co2 to water... do you guys feel that’s accurate? researchgate.net/publication/… fresh water systems $\endgroup$
    – JarredAwesome
    Commented Jan 19, 2020 at 19:35
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    $\begingroup$ When you use a typical air pump in aquariums, it is also pumping carbon dioxide from the air. I feel this is good enough. Surface agitation will certainly help to dissolve more CO2 and air of course, if CO2 level in air is way higher. $\endgroup$
    – ACR
    Commented Jan 19, 2020 at 20:26
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I am going the other way: I have a 125 gal with only 3 average size tropicals and numerous plants. I use aeration to get CO2 from the atmosphere into the water . As you said ; aeration promotes equilibrium between gases in the atmosphere and gases dissolved in the water.

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  • $\begingroup$ I was thinking of going that way too. I was considering doing a lily pipe. Apparently that adds co2 also. However, I decided to go pressurized co2. I’m just trying to figure out how much, of any co’2 my air pump is going to off gas $\endgroup$
    – JarredAwesome
    Commented Jan 19, 2020 at 22:48
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To add CO2, the answer is buffering as occurs naturally in nature (with carbonate rocks, for example). To quote a source:

An important breakthrough came when the researchers examined their results from the perspective of carbon dioxide in the oceans. In ocean waters with high buffering, that is, a greater capacity to neutralize acid, carbon dioxide and oxygen behave very differently than they do in waters with low buffering. For one, much more excess carbon dioxide lingers in highly buffered ocean water.

Researchers examined their samples and found that the degree of carbonate buffering controls the amount of excess carbon dioxide in surface freshwater. Alkalinity is a good marker for the degree of buffering and is an important consideration when looking at carbon dioxide in rivers.

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A precise answer to your question is : Yes. Adding surface agitation, or aeration to an aquarium with high levels of $CO_2$, causes the $CO_2$ to dissipate from the water.

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  • $\begingroup$ Does the level of co2 in the water have to be higher in the water than in the air? $\endgroup$
    – JarredAwesome
    Commented Jan 19, 2020 at 19:40
  • $\begingroup$ OP skip this answer as the result is history related. Of course shaking soda cans content lowers the CO2 concentration... See theorist and Farooq answer. $\endgroup$
    – Alchimista
    Commented Jan 21, 2020 at 8:08

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