Create a new Scratch project and delete the cat sprite by right clicking on the cat and then clicking on the 'delete' option in the menu that appears.

https://scratch.mit.edu

First let's add an underwater backdrop to our project from the backdrop library. Follow these steps to add in an underwater backdrop:

1. Click the Choose backdrop from library button (highlighted in yellow in the image on the right).
2. Find the 'underwater3' backdrop.
3. Select the backdrop.
4. Click OK.

Now let's add a fish sprite from the sprite library. Follow these steps to add the fish sprite:

1. Open the sprite library (by clicking the sprite library button highlighted in yellow in the figure on the right)
2. Scroll down and find the 'Fish1' sprite.
3. Select the sprite by clicking on it.
4. Click 'OK' to add it to your project.
   

Now add a shark sprite from the sprite library. Follow these steps to add the shark sprite:

1. Open the sprite library (by clicking the sprite library button highlighted in yellow in the figure on the right)
   
2. Scroll down and find the 'Shark' sprite.
3. Select the sprite by clicking on it.
4. Click 'OK' to add it to your project.

Now let's program the fish to swim around the tank. We'll also give the fish some instructions to shrink a little bit.

Click on the fish sprite in the sprite list (below the stage area) and then add the following code to the fish:

1. events when clicked 
2. looks set size to 50 % 
3. looks show 
4. motion set rotation style [left-right] 
5. control forever 
   1. motion move 2 steps 
   2. motion if on edge, bounce 

Next we will program the fish to disappear if the shark touches it! Add the following code to the fish:

1. events when clicked 
2. control forever 
   1. control if then 
      (put sensing touching [Shark] as the condition)
      1. looks hide