plastic fuel tank problem

[Utahboatnut]

Owned my crownline for four seasons now been a great boat. Filled up tank before long weekend and noticed a fuel smell next day opened access hatch and noticed fuel weeping out of fitting where supply comes out. Large nut with elbow threaded into it, the large nut is leaking. I tried to tighten the nut a bit but no luck,is this cast on the tank somehow or is there supposed to be an O-ring to seal. The only way I would be able to see is if I took out the sending unit and tried to stick my hand in there a bit, to feel whats on the other side of the nut. How would I be abel to tighten with a wrench and hold the inside fitting from turning?. Is there a sealant I could use around the surface of the nut and tank, it isn't under pressure but if the tank is FULL it weeps out now, never has before. I would rather not have to remove the tank to fix this any ideas.