It's my understanding that the bubbles moving through the water don't really add oxygen. They cause the water to move from the bottom of the tank to the surface. That's where oxygenation takes place. So no, I don't think it would do much in the sump.
As stated above, air stone bubbles cause the deeper water to rise and mix with the surface water where oxygenation occurs. Air stones do help but a good filter will do this a lot better by creating a stronger flow of water throughout the tank. There should already be a good water flow in your sump so it's most likely better to have the air stone in the tank.
Agree and disagree. If it's a powerful air stone It will help in a sump by reoxygenating the water before it goes back into the tank. It'll just help more if it's in the tank, but if the tank isn't short of aeration it doesn't really matter where it is.
If it's not a powerful air stone it's not really helping anyway as the others have said.
This is the sump that I have,
1st chamber under the bio-balls is where i'm thinking of putting air bubbles.the bio balls sit on top of the mechanical filter pad plate and the Triple Cylinder air Stone will sit under the multi purpose bio media chamber, by the time the water goes through to the return pump all air bubbles should be out going through all the baffles. https://static.wixstatic.com/media/f94a ... 1b~mv2.jpg