Leaky fuel line at the pump

That is a compression flare fitting. The threads should never be sealed. Nuts should always be finger started for three turns to avoid cross threading.

The nut compresses the line flare to the inverted flare fitting on pump. Always use two wenches one to hold back on the pump fitting, the other on line nut to tighten. I use both wrenches in one hand, holding the pump fitting still with thumb palm, and squeezing other wrench on flare nut, with fingers. Flare wrenches work better than open end because they engage more sides of fitting. It takes experience in feel to sense the crush for correct tightness.