In need of a temporary fix....

[Delphinus]

Where is it leaking from? Through the hole in the glass itself? If so then it's not sealed up good enough.

Tightening the nut may be all that's needed but be careful -- if you overtighten you can crack the glass and then you have a bigger problem on your hands. Depends on the glass. I would think that it's next to impossible to crack 1/2" glass (the bulkhead might snap first) but I can speak from experience (twice over) that you can definitely crack 1/4" glass by overtightening a bulkhead.

Personally I think if it was me I'd lower the water level, then try siliconing under the nut. You'll need to let it sit 24 hours to cure before raising the water level again. But it would be probably the least risky endeavour and it ought to fix the problem permanently.

HTH..