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I know that there have been extensive discussions on the topic of CO2 being or not being displaced by O2 but I'd like to ask that question again.

I was looking at a heavily pearling tank the other day and I realized that I can't explain why the O2 level is extremely high but the CO2 is 35-40.

Does O2 indeed displace CO2 out of the water? Or rather what is the dependency and the factors that come into play?

My question is partly motivated by the Oxygen need of the substrate.

--Nikolay
 

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CO2 does not displace O2 from solution, and O2 does not displace CO2 from solution. For all intents and purposes the two are chemically unrelated in any way. There are indirect relationships. Aeration and increased circulation tends to make both O2 and CO2 approach the values they would have at equilbrium with air. With respiration O2 tends to drop while CO2 increases. With photosynthesis O2 tends to increase while CO2 decreases. If you force CO2 to a high concentration while photosynthesis is going on then you get high levels of both CO2 and O2 and over some range of concentrations you may see some positive correlation between them.


Roger Miller
 

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I'm not sure I entirely understand what you're asking, but I'd like to for the sake of my attempts to understand chemistry :)

If a body of gas is 100% O2, and you add something else (in this case, CO2), then the percentage of O2 within the body of gas goes down, but not the quantity itself. This also means the body of gas had to expand, or it gains pressure. Now, that is just a basic physical deduction.

What comes into play is the capacity of a given space to hold a concentration of these gases.

I don't know if O2 molecules occupy the same physical "space" as CO2 in an aquatic system. The answer to the question of whether or not there is a correlation between CO2 and O2 in a tank would depend on the answer to that, because if a maximum concentration is reached (whether by one or both of the gases), and the molecules occupy the same "space", then they might affect one another's concentrations.

My shot at chemistry :)
 

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These are over simplifications of complex processes:

Error, Yes and No.

The difference between O2 and CO2 is how they dissolve. O2 is only in solution due to partial pressure from the atmosphere, CO2 actually dissolves. N2 is the same as O2 in how it dissolves, infact you can bubble N2 into a tank and displace O2 with it. CO2 breaks apart and becomes carbonic acid and and then carbonates and bicarbonates, how much depends on the GH of the water. In an aquarium devoid of everything but water, bubbling CO2 into the water would lower the pH but not force the O2 out.


Niko,

The substrate is a different beast as the O2 that can be found there is from the plant roots. CO2 and some other gasses (that are nasty) come from the respiration of bacteria and the plant roots. If you watch a tank the first place you see pearling will be in a stand of riccia, then the stem plants, then the 'rosette' plants. This is because Riccia has no root system while the other plants have to supply O2 to the roots. The water can be saturated with O2 but the rooted plants won't pearl until they have pushed O2 to the roots and the back pressure forces them to release the excess to the water column. Stems have the least developed roots so they pearl faster than rosette plants with extensive roots systems.
 

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Another good question resulting from aquarium observation.

What are the main factors that lead to loss or reduction of CO2 from aquarium water?

- use by plants during photosynthesis
- physical turbulence which ejects CO2 bubbles before they can be 'dissolved' into carbonates and bicarbonates
- increased water temperature which has both a physical effect of ejecting CO2 bubbles before dissolution and breakdown? of carbonates?

What are the main factors that lead to loss of O2 from aquarium water?

- use by plants during respiration
- use by organic matter in an oxidation process
- physical turbulence?
- increased water temperature
- an increase in N (nitrogen) for some reason

Are there things to add or subtract from these simplistic lists?

Andrew Cribb
 
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