Do you see any bubbles in the sump?

If not, it's not the sump design... but it could be improved like Jim said.



Your arrows are confusing me...
I'm going to assume that the skimmer section is where the tank waster drains into the sump... otherwise you could get microbubbles from the skimmer...

Check the return lines or replace them to see it that will work. I'm thinking cut off about 2" from the end that connects to the opposite side of the return pump.