Fish are water dwelling vertebrate animals characterised by fins, gills and scales they are adapted to living in water. A fish gets oxygen by passing water containing oxygen over a membrane rich in blood. The blood absorbs oxygen from the water and passes out carbon dioxide. Some fish can gulp air and absorb oxygen through their digestive system in emergencies.
Fish are designed to tolerate a range of water or salinity; if one type of fish goes into water with salinity beyond that range, it will usually die. Fish may also have specific ranges of water temperature or pressure which they can tolerate. They may be vegetarians or carnivores.
Fish range in size from under an inch to many feet in length. A variety of fish are suitable for captivity in aquariums for home display. Goldfish, minnows, guppies and some tropical fish are commonly seen in homes. Fish like cod, salmon and tuna are caught for their meat.
Whales and dolphins are not fish but air breathing mammals. Sharks do not have scales, but are in fact fish.