Originally Posted by sarahspins
Most of the inline diffusers need a fairly high PSI to work (35+).. I'm not sure an air pump is capable of generating enough pressure to do that. 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.
Good response - this was my thinking as well. Just add a spray bar and agitate the surface for a bit during lights out to get some gas exchange going.