she's not at a lake. she's at the spring path in route 214
You get it from Cynthia the Pokemon league champion.
In Pokemon league
The champion is Pokemon Diamond, Pearl and Platinum, is Cynthia. She has various Pokemon-types. She is the Sinnoh Champion. Her Pokemon are Spiritomb, Lucario, Togekiss, Milotic, Garchomp and Roserade. In Platinum, Cynthia has a Togekiss. In Diamond and Pearl, she has a Gastrodon.
Beat the pokemon league Cynthia is the champion
No
Cynthia the Pokemon Champion has a Milotic in Pokemon Diamond, Pokemon Pearl and Pokemon Platinum.
Constipated.
you go to lake valor, where you'll meet the Pokemon master Cynthia. she'll give you the secretpotion, which you spray on the psyducks to get past them.
She is the POKEMON CHAMP!She helps you through the story
Cynthia's grandmother gives it to you.
Cynthia elite 4
Get the SecretPotion from Cynthia