Many friends mentioned that I should have more instructions for my game, so they know what to do. Therefore, I would suggest some tasks below:
For Dory, Nemo & Shark, you just need to get familiar with the buttons, no tasks are required!
For Underground, you only can move Dory, and your task is to find out the hidden message from the helpers so that you can save Dory & Nemo!
For the fish tank, you only can move Dory too, and your task is to find out the hidden message from Tad so that you can help Nemo to escape (try not to be caught by the little girl ar!)
Thanks for my dear friends, again :)