Also make sure the inlet is not next to a bubbler or too close to the surface where it can suck in air.
The leak can be an ill fitting o-ring, so make sure you seat all of them. You might want to grease the o-rings with petroleum jelly for better seal.
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.