Pointing to your pet fish is a great way to introduce the baby sign for fish. Even if you don't have a pet fish, you can point to a picture of a fish in a story book.
Fish is signed with your right hand at chest level. Use the B hand shape with your palm facing left. Move your hand forward and across your body while bending your wrist left and right as if your hand were a swimming fish.
Discover more baby signs at the Babystrology baby sign language dictionary.