Gravel Thickness for effective biofiltration

All,

I have a 10 G unplanted FW aquarium with a power filter & 2 small gold
fish. I also have a bio sponge designed in the power filter. I have a
gravel bed comprising of 5-10 mm stones usually available at the pet
stores. My question is what should be the optimum depth of the gravel
bed to ensure that i supply maximum oxygen to these good bacteria to
colonize every area of my aquarium.

The aquarium has been up for more than 4 months now and has been doing
fine on all the critical parameters ..i do 30% water changes with
gravel vaccuming every week & my nitrates usually read below 10 ppm.

Appreciate every response.

Thanks
Kedar