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.
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Surf to Cianwood City, there is a pharmacy there and the guy inside will give you secret medicine.
cant
Cianwood Pharmacy.
You cant
you cant catch it in soul silver.