Ernie,
Scrap off the silicone around the hole. Make sure the rubber gasket and the inside of the tank is clean, place it under the flange and insert the whole thing from the inside of the tank. run a bead of clear silicone on the outside (back wall) where the bulkhead comes through the glass before you put the nut on, don't over tighten the nut. It should not leak.
Don't mess around with the glass.
Scrap off the silicone around the hole. Make sure the rubber gasket and the inside of the tank is clean, place it under the flange and insert the whole thing from the inside of the tank. run a bead of clear silicone on the outside (back wall) where the bulkhead comes through the glass before you put the nut on, don't over tighten the nut. It should not leak.
Don't mess around with the glass.