You have to talk to Cynthia and she will give you a secret potion to give to them
The path leads to Celestic Town.
Get A Pokemon That Can Use DeFog,and navigate your way celestic town.
At some point in the game, Cynthia will appear and ask you to take something to her grandmother in Celestic Town. After you do that, you'll get a potion called SecretPotion. You use this when talking to the psyducks blocking the path to clear them away.
Beat a Gym Leader, Cynthia talks to you, tells you to visit her grandma in Celestic town, and gives you a Secret Potion. Use it on the Psyducks and you can get past them.
Cynthia will give you a medicine to use on them and a gift to give to her grandmother in celestic town.
after you defeat crasherwake there is going to be a galactic grunt standing outside the great marsh you have to chase him until he wants to battle you. If you beat him cynthia will give you the secret potion then go to the psyducks in front of solaceon town and talk to the psyducks then they go away then go forward then you will need to use defog in a route then pass that route and you are in celestic town.
I don't have Pokemon Diamond,but my guess is up Solaceon Town,just keep on going north.But to get to Celestic Town,you have to beat a Galactic Grunt in Pastoria City.Then Cynthia gives you some kind of SecretPotion to give to the Psyducks.
At Valor lakefront, Cynthia gives you the secret potion. Give this to the psyducks by cafe cabin, and go down that road until you get to celestic town.
In Pokemon Pearl, the player must get the SecretPotion from Cynthia to clear a pack of Psyducks from the route that leads to Celestic Town. This is the only way to proceed forward.
You get HM03 Surf when you go to Celestic Town which is above Solaceon Town where the Psyducks are suffering from headaches. You will fight a galactic grunt when you reach Celestic Town, after that go into the ruins after you talk to the elder. The elder will give you HM03 Surf after you press A facing to the picture on the wall.
When you have revived the psyduck of their headaches, lots of trainers will battle you on your way to celestic town but you have to use defog then there will be a path and i think that you get the hm03 surf in celestic town but i am not certain because i did that step ages ago.
Cynthia gives you the potion to cure the psyducks headaches, then go to celestic town and defeat the galactic grunt, then go inside the cave and Cynthia's grandmother will give you the HM.