.. AN fitting leak / help please ..

The AN fittings don't usually leak when it's AN to AN if you tighten the crap out of them.

They usually leak more so when you screw (example) say a 3/8 NPT to 6 AN fitting into a fuel pump or filter in which I always use some Teflon but they do t normally leak where the AN male screws to an AN female unless you just did not snug them up enough.

Curious on another point.
You mentioned you just had a fuel cell installed.
By chance did you remove the anti slosh foam inside the cell?

I did not do that with my first go around and ended up gumming up my filter and my whole fuel system with deteriorated foam from the cell after about 8 months of use.

Of course I drained my fuel cell of fuel when I did this and. I noticed another problem.......underneath the foam, the morons never cleaned out any of the drill shavings when they drilled the holes for the welded in bung holes at the sump so I ended up removing the whole fuel cell and flushing out all of the aluminum drill shavings in which most settled at the bottom of my sump.
Just food for thought.

As far as fittings I've also had issues with boogie red up collars and not so great flaring jobs causing leaks.