if you have a power filter (hang on back type, I assume) rather than an
undergravel filter, then it's the bio sponge in your power filter that
is your primary site for aerobic (the good kind) bacteria colonization.
you want your substrate (gravel bed) to be as shallow as possible
(i.e. just enough to cover the bottom), since all you'd be doing by
adding more is creating the possibility of anaerobic (without oxygen)
pockets where bad bacteria will grow. this would not be the case if
you were using an undergravel filter, since the gravel has water being
pulled through it, acting as the colonization site, and I'm assuming
this is where your misconception comes from.

Kedar wrote:
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