Instructions
Talk with the Princess and she will tell you about her missing shells. There are nine in all and they are broken down in to three common, three uncommon and three rare shells.
Visit all the tanks of your neighbors in the game. Click on the treasure chests in the tanks. Each time you do you will find coins and possibly shells. The more neighbors you have, the better the chances are of you finding the shells. You may have to visit each neighbor several times in order to find them all.
Keep all nine shells plus the Seashell Temple, your prize for finding the shells, once you have completed the challenge.