Most fish are cold-bloded vertibrates (meaning their temperature is regulated by their environment and they have a backbone.) They lives in either fresh or saltwater (sometimes both.)
Fish have gills during every part of their life cycle. They can breathe by pulling oxygen out of the water when the water flows through their gills as they swim.
If a fish has has any limbs (because believe it or not some don’t) they are shaped like fins. The body of a fish is, for the most part, covered with scales. Fish reproduce in different ways, but most female fish lay hundreds of small egss that are then fertilized by the male. The body of a fish is covered in a protective slime that serves to help it move more easly through water and protects it from parasites and disease.