Could be a crack in the u-tube. Are they u-tubes or more like j-tube? I encountered this once with an eshops box. for some reason the j-tube was breaking suction. once switched to u-tube it was fine
I can watch the bubbles go through the tube from the box to the back.....looks like the water level in box is low and the splashing is causing the bubbles. I took one of the u tubes out and does great on one. I am starting to believe that that i am draining to fast.