Replacing a fuel line on a 1989 Nissan Pulsar?

This stuff you put in the tank was a waste of time and probably had nothing to do with this problem. This is an old car and probably all corroded underneath. You'll have to jack it up and drop the tank, and find out where the lines are rusted out. You'll have to fabricate some new sections and figure out how you're going to install it. It will be a hard and dirty job, I assure you, I've done this kind of thing more than once.