That said, an air stone itself doesn't actually add much oxygen to the water - what it does is create a surface disturbance and causes the water to flow upwards towards the surface, which is where the water is oxygenated, so adding air into your return line might not really do much.
If you can increase the surface agitation caused by your filter's outflow, that should provide more than adequate oxygen to your tank.
I've always agreed with that, but I also wonder how much oxygen is actually diffused into the water from the time the bubble leaves an air stone at the bottom and when it reaches the surface.
Anyone know? I'm in research mode now