Talk to Jasmine in Olivine Lighthouse first, and make sure you say 'Yes' when he asks if it is an emergency.
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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.
After you have defeated leader wake in pastoria city, Cynthia will give you a secretpotion, go to the psyducks and they will go away
you go to lake valor, where you'll meet the Pokemon master Cynthia. she'll give you the secretpotion, which you spray on the psyducks to get past them.
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.