Will an airpump increase the CO2 level in my aquarium?
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.
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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.
CO? is far more soluble in water than O?. The problem is the amount of CO? in air. Air is only about 0.0407% CO?. There is just not enough of it. Dissolved gases compete with each other so even though CO? is more soluble it does not compete for space with O? and N? that dominate air.
That said the ocean has a lot of CO? but there is a lot of ocean.
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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