The smallest fish in the world is the Paedocypris progenetica, a tiny fish that lives in the forest swamps in Sumatra and measures a mere 0.31 inches in length. According to the National History Museum, its existence was unknown until 2006, when it was first discovered.
This tiny fish is not only the world's smallest fish, but it is also the smallest vertebrate. It has a see-through body and at first glance looks like larvae. Its lives in water that is 100 times more acidic than rainwater with a pH of 3. The fish is a member of the carp family, and has oddly-made fins that are capable of grasping.