Air stones are not really needed generally.
They look nice but in a tank that size it's unlikely to be needed.
overstocked tanks result in the O2 in the water being used up. That and if your tank is too warm, the o2 will deplete faster.
Remember that PLants add O2 and convert CO2 as on land. And if you have any surface agitation, this will also re oxygenate the tank. As the water moves it will absorb o2 from the air. If you have a
trickle filter that makes bubbles, this will be more so.
The more surface agitation, the better gas exchange.
I have a bubble machine / light thingy in my 200l(55gal) but nothing in my 30l(10gal) as i have an overhead trickle filter.
edit - air pumps are noisy too! :P