Fish live underwater, in either salt or fresh water. They can only breathe underwater, but they use gills to breathe, not lungs like we do. Fish are covered in scales. Some people keep very small fish in their houses as pets. The biggest kind of fish is a whale shark, which is just a really big kind of shark. Whales grow a lot bigger than sharks but they are not fish, as they don't have gills and have to come to the surface of the water to breathe, just like we do.