Originally Posted by Melevsreef.com
A bubble trap is a narrow section within the sump that is made up of three baffles. The center one is higher than the two outer ones. The bubble trap should be located after the area that contains the most bubbles. Normally, that would be where the water enters the sump and your protein skimmer is located. Microbubbles abound in this zone, so the bubble trap would be set up next, to prevent these from continuing through to the refugium and/or return section. If you have a refugium next, you'll need one more baffle between the refugium and the return section. All four baffles should be the same height in this setup.
Flip that sump around. According to Melev ^ the bubble trap should be first.