I used to seal bulkheads in a very odd way. I put silicone on the plastic, then the rubber gasket, then more silicone on the other side of the rubber. Then when it is tightened to the tank it is basically glued in place and is VERY hard to break the seal...
However, in realizing my newbie mistakes, I have since switched paths on bulkhead installation. I still use silicone, kinda, but it is the 100% silicone magic lube instead. It is a synthetic grease that is sticky but allows the rubber to seat very nicely to the tank.
I have never really been able to get a bulkhead to seal right without some sort of silicone or grease. It just seems to work for me.