This fish sign looks a lot like fish swimming through the water. Take your dominant hand, and extend you hand with your thumb up and fingers together. Hold the hand out with fingers facing forward, and then shimmy your hand as you move it forward. Some people also sign fish using both hands with palms facing each other, as if swimming through waters together.
Use the animal signs to play a game with your baby. Show your baby the animal flash cards, and do the appropriate sign until your baby starts to be able to sign back.
All babies will show curiosity about animals and nature. Teach the sign for fish contextually if you are lucky to have an aquarium, or use storybooks and flash cards to reinforce the sign.