Help Removing Hose Barb from Poly Tank?

[Blue_Runner]

I suspect I have a ball valve in the hose barb. I want to replace it with a regular hose barb. Took off the fule line and I put a wrench on the hose barb itself. It kinda torques to the side and does not budge. I noticed a large metal nut at the base of it. Do I need to find a large wrench to hold this nut while I try to turn the hose barb? Or do I need to be turning that instead? Seems like that nut would stay put and the hose barb will just screw out of it. I put some lube on it yesterday.
My biggest concern is ruining the poly tank by trying to turn it too hard.......
p.s. Not much room to work in the inspection hatch.