It is clear that moving in water is more difficult than in air, so fish with a need to move quickly often have elongated bodies or are shaped like cylinders or torpedoes. This shape helps reduce the amount of friction caused by moving through the water and tends to make these animals grow fast. The exact number of fish species worldwide is unknown, as it is believed that many species of fish remain undiscovered. However, scientists estimate that the total number of fish species in the world is about 33,600. 

With more than 31,900 species identified to date, about 51% of all fish species are found in freshwater — that's more than 18,000 different species. And they make up a quarter of all the vertebrates in the world. Of course, those are just the species that we know of. So, out of these species, which is the fastest freshwater fish?