So is the leak coming from around the outside of the bulkhead? i.e. tank water that is leaking? Or is the leak coming from the inside of the bulkhead and it's pressurized water from the pump that's leaking?
#1) no silicon glue ever.
If it's just tank water leaking by the rubber gasket then most likely the gasket has crud on it or something like gravel, sand, dirt, is between the gasket and the glass
or the gasket and the bulkhead flange. Remove the bulkhead and clean the area around the flange and the area around the hole and make sure the rubber gasket is free from grit and dirt. Just a quick wipe with a paper towel will do. Then tighten that mother down with channel locks. The gasket needs to compress against the glass and the bulkhead flange.
Tip: The only part turning when tightening a bulkhead should be the nut on the
outside of the aquarium. The actual bulkhead does not move. If the bulkhead moves while your tightening then the rubber gasket can get a slight fold or crease in it causing a small leak.
If your leak is on the inside i.e. high pressure water from the pump return then it's just a pipe thread issue. You need to remove that fitting and tape it with teflon tape and install it again tight. Ya got to be careful with this because over tightening could cause the bulkhead to split. If you have a schedule 80 bulkhead (gray in color and beefy) then you wont have to worry. If you have a cheapo black plastic type then it may crack or has cracked. If it's cracked you need a new one. Replace it with the same type because the hole size may be different if you change to a schedule 80 type.
Hope this isn't too much info
