What is causing toilet supply water leaks above the coupling nut?

Question

In my toilet, when the shut off water valve is open:

Water leaks above the coupling nut:

If I close the supply water valve and the tank is full, there is no leakage. What could be causing the water leakage? The water supply line is less than a year old. I also taped the connection where the coupling nut screw over and it is still leaking.

Is it possible that the water supply line is broken at the nut? Should I get a new one? The supply line I have has a plastic nut and is flexible.

Solution:

There was actually a hairline crack in the plastic tube that comes out of the tank to the outside. The plastic pipe was replaced with a brass one to fix the issue.

Answer 1

The washer under the nut is probably leaking and the easiest/best solution's would to be replace the pipe.

Why

The reason it only leaks when you have the supply on is that there is no significant back pressure on the fittings from the tank as the tank has an air gap and the filler is at the top of the tank not submerged at the bottom ( which would cause back pressure)

P.S. DON'T use mutigrips, use a spanner or shifting spanner, on a plastic nut (or even brass) you can deform the nut by the crushing force of the mutigrips as you tighten the nut.

Answer 2

Yes, that is the supply line. Believe it or not, in those types of fittings, the tighter you tighten the connection, the greater the chance of a leak. They are typically compression fittings using either a brass compression ring or a rubber type of washer that compress when tightened. They should be hand tightened then snugged (1/4 turn more) with an open end wrench, adjustable wrench (shifting spanner).