I generally use both teflon + silicone, and do not tighten much at all. I have cracked a fitting once (not on aquarium though). You do it once, then usually never again, since you know how hard to tighten. A bigger concern for me on bulkheads was squashing the rubber gasket out by over tightening. That happened a couple of times. Usually on cheap bulkheads with thin gaskets. I stopped using those as well. Lots of silicone instead.