Air pumps (let's say with 200 l/h) slightly increase the level of oxygen in the water simply by making bubbles rise from the bottom of the aquarium to the water surface. Can I expect an (even minimal) effect for the CO2 level as well?

I know that concentrated CO2 from a bio facility or a bottle is disolved using a flipper or a diffusor and that's the way to go for reliable CO2 supply (otherwise there'd be no CO2 facilities for aquariums), but since pumping masses of air and therefore increasing the level in the water has an effect for the gas O2 it might have one for the gas CO2 as well.

I have a 60 l freshwater aquarium with 35 cm distance between the gravel surface and the water surface.

  • I've never heard of deliberately adding co2 to an aquarium; if someone has a pointer to a citation and explanation I'd appreciate it. Unless you were deliberately trying to reproduce acidic swamp-lije conditions I can't imagine why one would want this. All I've ever seen is straight aeration. – keshlam Jan 8 '17 at 18:15
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    We deliberately add Co2 to high tech planted tanks all the time. This has been a common practice for at least 20 years if not more and significantly increases growth of plants as well as allows plants to out compete algaes under high light, high nutrient conditions. Depending on the plant, the growth difference can be greater than a factor of 10. This does come at the potential expense of livestock health, since CO2 is toxic to animals in high concentrations and if enough is added will displace O2 in an aquatic environment. But, yes CO2 is routinely added to high tech planted tank. – Jestep Jan 8 '17 at 19:44

No, aeration will typically cause CO2 degassing unless the room the tank is in has substantially elevated CO2 levels. I've personally tested the differences in CO2 concentration when aerating and not aerating, with other factors controlled, and CO2 levels go down or have negligible change, likely depending on the initial concentration.

I do not know the specific chemical mechanism for this, but I suspect that O2 is far more solvent in water so the increased surface area is consumed by O2 absorption rather than CO2 absorption. The increased surface area due to aeration is the primary mechanism for this as far as what I've read. Additionally, CO2 levels remain fairly consistent due to surface tension, again with aeration, that surface tension is broken, allowing CO2 to degas. If I can find a specific reference to this phenomenon, I will update this post. But at least anecdotally in several decades of keeping planted tanks, aeration and increased surface area through mechanical means, decreases CO2 concentration and increases O2 absorption.

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  • O₂ is not far more solvent than CO₂. O₂ has no polarity. CO₂ is about 50 times more solvent than O₂ in water. – paparazzo Jan 9 '17 at 19:19
  • Ok, thank you for clarifying, then it is strictly an issue of availability? – Jestep Jan 10 '17 at 16:23

CO₂ is far more soluble in water than O₂. The problem is the amount of CO₂ in air. Air is only about 0.0407% CO₂. There is just not enough of it. Dissolved gases compete with each other so even though CO₂ is more soluble it does not compete for space with O₂ and N₂ that dominate air.

That said the ocean has a lot of CO₂ but there is a lot of ocean.

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Maybe, depends on how many vigorous plants you have. If you have no plants it will make little difference. Many growing plants will be absorbing CO2 from the water and so the water will have less than equilibrium level of CO2 relative to the air, and an aerator will add CO2. I had a 75 g salt tank with a lot of Caulerpa (algae) in it - I sold it to pet shops to get rid of it. At night the pH would get more acidic because of the increased CO2 saturation (down to about 8.2) with aerators running. During the day the plants consumed so much CO2 that the filtering and aeration could not replace it fast enough and the pH went to about 8.7. The numbers don't sound like much, but consider that is changing the pH of about 700 lbs of water. Too hard for me to calculate how many pounds of CO2 that required.

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  • I want to further confirm that a pH change from 8.2 to 8.7 is indeed a lot - pH scale is not linear, but logarithmic which means that even a seemingly small nudge means a lot. A solution with pH of 8.7 is over 3 times less acidic than a solution with pH of 8.2. – lila Apr 24 at 19:56
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    I didn't check my log book , might have been 8.2 to 8.6. And it was essentially sea water with 500 ppm calcium. – blacksmith37 Apr 25 at 2:18
  • Oh, what I wanted to mean was to just confirm your post's validity as it already was, not to challenge it nor ask for more evidence. I did that because you wrote about the change of pH from 8.2 to 8.7 not sounding like much, so I wanted my comment to serve as a confirmation that it's OK and it makes sense, and also as an explaination of pH scale principle for anyone who would happen to read your post in future. – lila Apr 25 at 13:42

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