What size is your sump? What is your return pump?
If the microbubbles are not from a skimmer, and they are making it from your sump to your display, then your sump design is not allowing enough time for the bubbles to rise to the surface and pop, before hitting the return pump.
This means your sump is of poor design, or your return pump is too powerfull for the size of sump.
By poor design I mean not enough baffles to act as a bubble trap. How many do you have? What is the spacing between them?
Sometimes the turnover rate through the sump is just too high to be dealt with by baffles. In this case I would throttle back your return pump. Don't try to accomplish too much water flow through your sump return.
You can also play with the angle the return line empties into the sump, try putting a 45 degree elbow on the end and have it enter just below the water line.
A filter sock is a patch, it doesn't address the problem. It may solve it, but they are notoriously undermaintained and end up causing more harm than good.
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