Do you have bubble trap baffles in your sump? If so, are you seeing bubbles get past them to your return pump? If there's bubbles in your sump getting to your return pump, try adding baffles or even a sponge before the return pump. If you use a sponge, make sure you rinse it at least once a week. Otherwise, it'll become a nitrate producer. If there are no bubbles getting to your return pump, the cause is between the return pump and your display. Another cause could be low water in the return area allowing air to get sucked up by the return pump and added to the water. I have that problem if I allow enough evap.