Get a filter sock (25 micron) and place on the end of the overflow discharge in the sump. get a couple as you need to clean them every week. Also could add solid staggered hieght baffles after the discharge area and before the return pump pick up. Look in DIY section on Sump builds.
The other possible source is the discharge from the skimmer, I also use a sock there on my setup and no micro bubbles
Ron