You need the current password. Take the secret message from the Maid's room (castle first floor left) and go to the upper floor of the mill. The girl will give you a Cosmic Symbols card. Go to the fountain and press the three symbols, and the sun.
It will help you go down to the Mordred club.
The plaque is on the fountain outside. It has various planets, stars, and moons on it. (You will need to take the Secret Message to the upper floor of the mill to get the password symbols.)
Go to the fountain in the town square. Click on the plaque, and enter the three symbols and the center sun.
It is next to the Mordred Museum at the entrance to the island.
Once you get the Cosmic Symbols card from the girl at the mill, you can enter the symbols in any order on the fountain plaque, then press the center Sun to open it.
you can get the coin in astro knights island on the fountain
It is a "brass plaque", on the fountain in the village. The same place where you got the gold coin to enter the museum. Once you have the Cosmic Symbols card, you can open the secret door behind the plaque.
The Cosmic Symbols password card lets you enter the room under the fountain in the village. Pressing the three symbols (and the sun) on the fountain plaque will open the secret door.
If you climb the fountain, it is in the water halfway up.
There is a gold coin outside in the fountain.
In order to get the coin from the fountain in Poptropica Astro Knights Island, you must go to the fountain and look for the shiny coin. Once you spot it, you walk over to it and it will be added to your dock.
The Cosmic Symbols card lets you access the hidden panel on the town fountain. Pressing the three symbols (and the center Sun) will open the plaque.