Looks like about four inches of water in your sump. Not sure what size pump you have but if my sump gets to four inches my rio will pull in air and send it up to the display.

Unless I miss my guess you have two pumps in there, one is your skimmer and the other is your return pump... that compiled with the fact that it looks like you're sending your skimmer water back into the sump makes me think you have a lot of movement in the sump. I think you may be sucking up air bubbles with all that movement.