Poodles;1086318 said:It's sealed with corrosion...
Spray it with PB Blaster, and WALK AWAY. Let it sit overnight even...
Shytheed Dumas;1086351 said:^^ This and a vice grip.
^^^ Agreed. Since you pretty much rounded off the nut, go after it with vice grips; and then once you got it apart, you can figure out what you need to do to correctly reinstall the new line.
BTW, if the nut that's rounded is the nut on the hard line, then correctly means:
1. Replace that 6' of hard line with new line.
or
2. Cut the hard line back just a little bit, and pull the rounded nut off the hard line. Get a new nut, and put it on the hard line; then flare the end of the hard line with a flaring tool. When you're done it'll be good as new.