Originally Posted by KingOfTheFeeshes
That's just not true... If you have much higher CO2 levels, your oxygen level may stay the same - but is lower in relation to the oxygen. As air is a solution, this is all distributed evenly. CO2 might not get rid of the oxygen, but it most definitely displaces it.
If you're correctly dosing your CO2 for your tank, you're also effectively asphyxiating your beneficial bacteria via this method of diffusion.
If CO2 displaces O2, where does the O2 that is displaced go? I don't believe this is true. Unless you are adding a lot more than 1% of anything to the air It shouldn't have any effect on the O2 percentage in the air - about 20%. And, we only add perhaps .3-.4% CO2 at the most. Furthermore, lots of people run their CO2 through the filter, without adverse effects, except it is possible to run into cavitation in the pump if you get too big a bubble of gas at one time.