Pokemon Diamond; Psyduck Potion

Question

Im stuck were do you go to get this potion cos ive been stuck for ages

Answer 1

After you beat the Pastoria Gym Leader, there will be a blue haired grunt standing by the entrance to the Great Marsh. Once you hear what he has to say (eavesdropping) then you will have to chase him to the right then you will talk to him again. Then he will run off again then you keep chasing him down by the lake front. After that you will keep chasing him through the condos (valor lakefront) and then you'll have to battle him then he will run out of breath and he will walk forward you then follow till you see Cynthia standing by the 2 men by the trees talk to her and she will then give you the secret potion for the Psyducks! Anything to help Pokemaniacs! :)

Answer 2

and after you talk to the galactic grunt again, you have to battle your rival. :)