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.
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You get the potion from Cynthia at Valor Lakefront. Then go to route 210 where the psyducks are.
After you have defeated leader wake in pastoria city, Cynthia will give you a secretpotion, go to the psyducks and they will go away
I don't have Pokemon Diamond,but my guess is up Solaceon Town,just keep on going north.But to get to Celestic Town,you have to beat a Galactic Grunt in Pastoria City.Then Cynthia gives you some kind of SecretPotion to give to the Psyducks.
Beat a Gym Leader, Cynthia talks to you, tells you to visit her grandma in Celestic town, and gives you a Secret Potion. Use it on the Psyducks and you can get past them.
The Psyducks are right above Solaceon City.