Air stones and air pumps do not directly add any oxygen to speak of to the water, the oxygen is added by gaseous exchange by surface agitation, as long as the surface ripples the oxygen goes in be it air stone, filter outlet or power head.
I have 3 returns from my external filter and the sump rippling the water surface plus a large koralia pump pointing at the water surface for massive movement and surface agitation.
Many owners see their rays playing in bubbles but mine paid it no attention that I saw so I took it out as air pumps are noisy and the pipes can be ugly.