You may have a small chip in the glass so that the rubber gasket does not form a seal with the glass no matter how hard you tighten the bulkhead. What you could do is run a bead of silicone around the bulkhead (on the flange side) and tighten it down. Of course you would have to leave the silicone to dry for a few hours.
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32"x32"x20" Cube-ish tank
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