We have a HOB overflow with 2 removable U-tubes moving the water. Once one of them kept building up bubbles b/c we put it in backwards (one side is slightly longer than the other)...other times we just kinda move them from one side to the other until we find the "sweet" spot. There is only like an inch of play but it makes a difference whether the tubes are pushed to the left or right side...
Not sure if this is of any help
but I tried...