I reccommend creating each fish in the library.

Check out this selection of fish or find your own fish.

I will demonstrate how to create a fish and move it across the screen and then create several fish moving in different directions.But for now, let's just create our fish, one by one in the library.

It might help to get a photo of a fish. I will create a fish from this photo. You can bring the photo into your document and create a new layer and trace it or draw from a picture you are looking at or just dream one up from your mind.
To begin while in the stage go to the library.

1) >Insert >New Symbol
2) Give it a name and select Movie Clip. Press OK.
3) Drag a photo in or skip to step 6.

4 ) Create a new layer. >Insert >Layer
5) If there is more than one layer,
6 ) Draw the fish.

Go back to step 1 and create another fish.

7) When ready to bring the fish into the movie, click on scene1, drag in the fish to the stage then drag it to the side of the publishable document. Be sure each fish is brought in on its own layer.
4) Save your work.