I don't know how much oxygen exchange can occur on a closed system.
To get better oxygen levels in the water column you can add a power head to agitate the surface or an airstone which will also agitate the surface which will aid in oxygen exchange as well as help oxygenated water to circulate to the bottom of your tank where some fish like to hang out.
I run a single airstone on all of my smaller tanks, 2 on my 46 gal
. and an air curtain about 5" long on my 75 gal
PS: keep in mind that the filter is not the only place the bacteria live and bio-filtration takes place throughout your tank.
The bacteria also lives in your substrate, on your glass and on ornaments in your tank.