Cynthia will give it to you.
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The SecretPotion gets rid of the Psyduck on Route 210.
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.
You get the secret potion by talking to the pharmacist in Cianwood City. His house is in the south, south east of the gym.
cianwood city
A bit up from the grand lake hotel, you will find Cynthia who gives you the secret potion. Also it will be needed to get to Celestic town, you use it on the Psyduck at route 210. Hope it helped!!