i think you go to the psyduck next to the moomoo milk place then the Pokemon league champ will show up and give you it
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.
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
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.
Cynthia will give it to you.
Valor Lakefront
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
In Cianwood city there should be a Medical store up north of the gym and the man inside the store will give you the secret potion.
You need the Secret Potion, and you can get it at the Cianawood Pharmacy.
Yes, beating the 7th Gym does not effect getting the secret potion.
Go to Pastoria city, and by the Great Marsh entrance you should see a galactic grunt. Keep chasing him until he stops and fights you. After you beat him, Cynthia will come and give you a secret potion. If you use it on the Psyduck, they will move.
Ok, after you get the Secret Potion from Cynthia, proceed on Route 210. That should take you to the Cafe Cabin. Walk to the upper left corner, where the Psyduck are. Press "A" when it asks you if you want to use your secret potion. Note: even after you use the Secret Potion it will still remain in your Key Items section of the bag Hope it helped!!
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The SecretPotion gets rid of the Psyduck on Route 210.