Try greasing the top o-ring with Vaseline for better seal.
Also, make sure all intake joints are tight - esp. the connection between the hose & the lid (which I found to be easily loosened), and that the intake is not sucking in air somewhere (bubble wand, etc.)
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.