I hesitate to use silicone.
The silicone is sooooo slippery that when I tighten the bulkhead the rubber gasket gets squeezed out from between bulkhead and the glass. I start without, and then if it's still leaking I may try using a LITTLE silicone.
The water depth makes a big difference. What might work with a couple of inches of head in the plumbing system won't work at the bottom of a 2 foot tank.