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.