How do you get the Psyducks out of the way Pokemon Diamond?

After you chase the Team Galactic Grunt to Lake Valor, Cythia will give you the Secret Potion. - SkyFly45

First, defeat Crasher Wake. When you get out of the Gym and talk to the Team Galactic Grunt, he will run away. Keep chasing him until he turns around and battles you. When you beat him, Cynthia will come and give you the Secret Potion. Then talk to the Psyduck and choose YES. Then they will walk away.