go to pastoria and beat the gym leader there.(you have to beat veilstone first)chase the galactic guy to lake valor and beat him. Cynthia will give you medicine to cure the psyduck and go to celestic town.
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The SecretPotion gets rid of the Psyduck on Route 210.
Just keep killing them in a row and sometime it will apear.
As far as i know it evolves into Golduck at lvl 33
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.
After you beat the gym leader in Veilstone {or Hearthome i cant remember} Cynthia should give you a secret potion. Then you go to the Psyduck and you push A. Then you can carry on from there. I hope I helped!!!