[fishytime]

Originally Posted by gregzz4

I wish you luck getting it out.

You say the nut is in the overflow ..... I hope this means the bulkhead goes into the overflow from the tank. If it exits the overflow to air, then it was installed backwards and this may be why it leaked.
The gasket goes on the wet side and the nut goes on the dry side. The reason for this is how it seats to the gasket.
Anyway ....
Once you get it out, I'd recommend you buy a new replacement and install it dry. Just past hand tight is a starting point. The gaskets do not like to seal more than once, so make sure you are finished playing with the plumbing before you tighten it.
I learned this when I tried to turn one last month. It would not re-seal.

Silicone may work for some, but it's not recommended as it can cause the gasket to squish out.

+1....previous owner installed the bulkhead wrong....... a good general rule is to replace any bulkheads when buying a used system.....or at least the gaskets...I would also add that when you instal the bulkhead dry, make sure the glass on both side of the hole are squeaky clean