You don't need an air stone. If your filter moves the water round the tank, that's all you need.
Oxygen gets into the water at the surface. As water moves round the tank, oxygen rich water is pushed to the bottom of the tank, and oxygen depleted water is brought to the surface where more oxygen enters the water. This provides enough oxygen, excpet possibly if the temperature is raised to treat whitespot or medication has been added to the tank, both of which reduce the oxygen carrying capacity of the water. In these cases, moving the filter outflow so it causes ripples on the surface is all the tank needs.
Airstones do not directly add oxygen to a tank. They help by churning the water up, as does a filter