I'm fitting my Durso pipe today, when I originally screwed in the original overflow pipe to the bulkhead, after tightening I could still turn the pipe and bulkhead. Should it not be tight enough to not turn on the glass? Hope this makes sense?
I never silicone mine. If I keep the lip of the bulkhead, then the one gasket between that lip and the tank's inner surface is sufficient. No gasket on the bottom of the tank... The gasket presses against the lip hence no water can leak.
If you cut off the lip, or use a bulkhead without this lip to thread something on the inside, then I use two gaskets. I also use teflon tape to seal the junction between the nut and the thread on the inside. This helps prevent water from leaking through the threads. If any water do manage to leak the gasket and teflon tape at the bottom provides a second layer of sealing. Maybe not perfect, but it works for me.