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I had a tank I used strictly for inhabitants, and I've converted it to a planted tank.

Now I have an air pump into it providing oxygen. I know that oxygen removes any co2 traces from the water.

My question is, if I am injecting a lot of CO2 into the water, will injecting the same amount of oxygen defuse excess co2 from the water, thus canceling each other out?
 

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There are several misconceptions that should be cleared up.

An airpump does not "provide" oxygen; the air bubbles it creates will minimally diffuse into the water, barely raising the dissolved oxygen level. The bubbles it creates, however, does agitate the surface, causing the surface area to increase, and thereby allowing greater gas exchange to occur. This means that oxygen will be able to diffuse from an area of high concentration (i.e. the air) to an area of low(er) concentration (i.e. the water in your aquarium).

Also, oxygen does not "remove" CO2 from the water. Injecting one gas will not force the other out (i.e. injecting pure oxygen from a tank would not eliminate carbon dioxide, and vice versa). For example, you could be injecting pure oxygen into your tank, but your fish could still die of carbon dioxide poisoning if their levels were high enough.

The disadvantage of using an airpump lies in the excessive surface area it creates. Because of the extra surface area, there is more gas diffusion. This means that the carbon dioxide will diffuse from an area of higher concentration (i.e. your aquarium water) to an area of lower concentration (i.e. the atmosphere). As a result, using an airpump and injecting CO2 at the same time are counterproductive.

To answer your question; if you are injecting CO2 into the water, and you are also injecting (pure) oxygen into the water (i.e. via an oxygen tank; an air pump does not inject oxygen into the tank!) and both are being fully dissolved and not agitating the surface of the water, neither gas will force the other out of solution.
 
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