Leaking Bulkheads

No, that is a common misconception. The gasket goes on the flange side, regardless if that is inside or outside the tank. In normal installations the flange is inside the tank but often you see someone needing that extra bit of room aso the install the bulkhead with the nut and majority of the plastic inside the tank. Setting a sump next to the tank is one example.

Anyway the point is the gasket always goes against the flange whether its wet or dry. If you install the gasket on the nut side, water has a clear path along the threads and will usually leak.