We are homeowners attempting some DIY projects in our bathroom. We are replacing a leaky supply line on our toilet. Haven't gotten too far, because we can't get the old supply line disconnected. I attached pictures. I am not sure if I have the names correctly, but the Coupling Nut (the larger one below the locking nut) won't budge. Should that locking nut be tight again the tank, then loosen the coupling nut? We have tried using 2 wrenches, but the nut won't budge and we don't want to crack the tank.
The large nut snugs the copper riser to the end of the fill valve.
The thin nut secures the fill valve in the tank.
These nuts are upside down. So make sure you are loosening them, not making them tighter. Just undo the large nut and it will drop down.