Hmmm. I'm not familiar with the 14 gallon but I'm sure the bubbles are being introduced in that middle chamber. I have the live rock stacked up high enough in my middle chamber to creat a trickle down affect rather than the water dropping from the tray down to the water level in the middle chamber if that makes sense...
The best I can say is watch thru the glass where your return pump is and see if you see any bubbles entering that chamber either from below or above. That has to be where it is picking up the air.