You get the secret potion by talking to the pharmacist in Cianwood City. His house is in the south, south east of the gym.
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you go down to the house next to the Pokemon center and inside is this guy, talk to him and he will give a secret potion.
Cynthia will give it to you.
cianwood city
The SecretPotion gets rid of the Psyduck on Route 210.
Pokemon league champion Cynthia will give it to you when you run into her at lake valor(near pastoria and sunyshore)