The Cosmic Symbols password card is given to you by the girl in the upper loft at the Mill. The symbols are crescent moon, Saturn, and star. It doesn't help to know this because the fountain plaque will NOT open unless you have acquired the card.
To open the plaque, press the crescent moon, Saturn, and star, and then the sun.
In the fountain. you have to find the password first.
It is next to the Mordred Museum at the entrance to the island.
you can get the coin in astro knights island on the fountain
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.
There is a gold coin outside in the fountain.
If you climb the fountain, it is in the water halfway up.
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.
Jump up on the fountain (a gold coin is about halfway up on the right).
The message doesn't say that there are papers at the fountain. The message will get you the PASSWORD to the hidden fountain entrance. Once you have the Princess's note from the maid's room (first floor of the castle, left), go to the mill (upstairs) and a girl will give you the password symbols for the fountain. (see related question)
press the moon button then figure out the combination
Jump to the second level where you see the sparkle.
its in the fountain