How big is the biggest fish in the world?
The whale shark, Rhincodon typus, is a slow filter feeding shark that is the largest living fish species. This distinctively-marked shark is the only member of its genus Rhincodon and its family, Rhincodontidae (called Rhinodontes before 1984), which is grouped into the subclass Elasmobranchii in the class Chondrichthyes. The shark is found in tropical and warm oceans and lives in the open sea. The species is believed to have originated about 60 million years ago.[2]
http://en.wikipedia.org/wiki/Whale_shark
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The whale shark is up to 46 feet (14 m), weighing up to 15 tons. The average size is 25 feet (7.6 m) long It is the largest fish in the world. Females are larger than males (like most sharks).
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it depends. the biggest fresh water fish at the moment is the giant catfish, the largest being 9m and weighing 646 pounds, but if you want salt water then the obvious answer would be the whale shark.