Well I'm still thinking about your question.
If this is just a plain tank or tub sump, without any dividers, you're probably seeing the micro bubbles coming out of the overflow hose/tube, entering the sump and getting pumped back up into your display tank. Is this what is happening ??
If so, you could get a sheet of egg crate and a sheet of acrylic, and cut each to fit across the sump, making sort of a divider or chamber for your overflow hose to enter. The egg crate would extend all the way from the bottom of the sump up to say a few inches from the top. The acrylic piece would be somewhat shorter in depth, but cut even with the top of the egg crate, allowing a inch or two of open eggcrate at the bottom.
Connect both pieces together with plastic wire ties or some reef safe epoxy/sealent.
What you want is a divider that keeps the bubbles in this first chamber, but allows water to flow under the divider to the rest of the sump.
Position your overflow hose towards the top of this first chamber, still underwater, but not down at the bottom.
Bubbles will rise given time, and we're trying to keep them in this chamber allowing them to rise, sort of buying a little time before they make it over to your return pump.
This is just an idea remember - maybe someone else has a better one ?
You could play around with a divider without having to shell out too much money. Eliminating the micro bubbles from the start would obviously be the best.
Just a thought JG24
Brian