easy
go to the left of moo moo milk shop, then give the secret potion to the psyduck :)
from Cynthia
You get the secret potion by talking to the pharmacist in Cianwood City. His house is in the south, south east of the gym.
The Secret Potion is a Key Item which can not be discarded, sold or used until the required event with the troubled Psyducks.
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!!
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 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 give it to the psyduck
Yes, beating the 7th Gym does not effect getting the secret potion.
cianwood city