If you're playing Diamond, Pearl, or Platinum, head north from Solaceon Town to the Psyduck blocking the road.
If you are Playing HeartGold or SoulSilver, head to the top of the Lighthouse in Olivine City and give it to Jasmine.
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The serum you are talking about is actually called a secretpotion. You have to go to Cianwood City's pharmacy. Once you talk to the clerk, he'll give you the secretpotion to give to the Ampharos in the lighthouse.
You can only get the SecretPotion after you have talked to Jasmine in the Olivine Lighthouse. Then she will ask you to get Ampharos some Medicine. You go to Cianwood City and talk to the guy at the Pharmacy and he will give you SecretPotion for Ampharos.
The SecretPotion gets rid of the Psyduck on Route 210.
Talk to Jasmine in Olivine Lighthouse first, and make sure you say 'Yes' when he asks if it is an emergency.
After you have defeated leader wake in pastoria city, Cynthia will give you a secretpotion, go to the psyducks and they will go away