With plants in an aquarium and no CO2 added, the concentration of CO2 goes below what it would be in water without plants/algae, as it is consumed by them. Surface agitation helps remedy the depleted CO2 state that the water is in by exposing more of it to the air (just like stirring sugar into a drink helps it dissolve faster than simply letting it sit at the bottom.) That doesn't mean that the CO2 concentration will be the same in the air and in the water as its solubility in the two solutions (the gaseous solution that is the atmosphere and the liquid solution that is tank water) is very very different. This is even more pronounced with something like oxygen, which dissolves wretchedly badly in water, so much so that it is 21% of the atmosphere but only 0.001% of water that is completely oxygen saturated.
If you're adding CO2 to a tank, then surface agitation tends to remove it from solution. Even so, it's typically desirable for you do have surface agitation to help break up films and keep oxygen levels high.
Another way to think of this is that if your idea were right, the water in an aquarium would be 78% nitrogen by weight, 21% oxygen, and 1% argon, with no room for the water left over.
ETA: elemental flub
Last edited by jasonpatterson; 08-06-2011 at 09:29 PM.