• a fish is an aquatic vertebrate animal
• it is covered with scales, equipped with two sets of paired fins and several unpaired fins
• most fish are "cold-blooded"(ectothermic) à their body  temperatures vary as ambient temperatures change
• fish are abundant in most bodies of water
• they can be found in nearly all aquatic environments, from high mountain streams to the abyssal and even hadal depths of the deepest oceans
• fish exhibit greater species diversity than any other class of vertebrates (31,900 species)

source: www.wikipedia.com