Actually the addition of oxygen to the tank has no effect on decreasing CO2 concentrations. CO2 will equalize with the CO2 of the atmosphere depending on your
pH and KH. The only way it will diminish CO2 levels is if you are adding CO2 to the tank in which case the surface agitation of the bubbles allows the release of the CO2 at a greater rate until equillibrium occurs, which is never.
Depending on the size of the bubbles, the main benefit is surface agitation which increases the gas exchange rate. Power filters will cause enough agitation to eliminate the need for an air pump (depending on tank population). It is dissolved oxygen that the fish utilize.
Nitrogen gas is the end result of the nitrogen cycle which is difficult to achieve in freshwater aquariums as opposed to marine aquariums, but is
desirable. It is best to "house" both types of bacteria to help complete this cycle. When nitrogen gas is not produced, it is because nitrate is not being converted (hence why water changes are important). When you complete the cycle, nitrogen gas is released to the atmosphere and does not build up in the aquarium (depending on your altitute, which is not an issue for anyone living on Earth). The nitrogen cycle, simply put, is the conversion of ammonia, to nitrite, to nitrate to
nitrogen.