Sounds like you just have too much flow through the sump for the ait bubbles to actually rise out of the water column before being sent back up to the display. Might want to try using a good filter sock on the overflow line, that will give any bubbles more of a chance to escape before passing through the sock. Just remember this will act as a sort of mechanical filtration and will need periodic cleaning. If you go this route use a decent quality sock, one of the thick wooly type.