you go to pastoria city look by the great marsh entrance and you should see a galactic grunt. chase him. he will resist battle until he is too tired to go on. fight him and beat him. then Cynthia should come up to you and give you the secret potion. it is a key item. go up to the psyduck and press A. it will ask if you want to use the secret potion. say yes. they will move and you can go to celestic town.
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The SecretPotion gets rid of the Psyduck on Route 210.
There is no Kecleon on Route 210.
After the explosion at the Great Marsh, follow the Cyrus and beat him. Then Cynthia will give you the SecretPotion, which makes the Psyduck's heads feel better and makes them move.
I've answered this questions plenty of times before. When you reach a certain town, Veilstone I think, Cynthia will give you a secret potion in which you can give to the psyduck on route 210, and they will go away.
Route 210 from Solaceon Town. pokemon breeder kahill past the twins