Teflon tape would work fairly well, but I usually silicone all the way around the bulkhead (on either the inside or outside.. doesn't really matter if the seal is completed) before I let 'er rip. With a good bit of tightening it's basically watertight, but it's also very easy for a small leak to still persist, and screwing that bulkhead on really, really tight can also stress the glass (mind you, I'm thinking of my 10 gallon fuge, not a behemoth display).
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