The freshwater giants

It is said that the largest freshwater fish in the world lives, of course, in the largest freshwater river in the world - the Amazon River in South America. Arapaima, also known locally as Pirarucu, Arapaima gigas are believed to be the largest, exclusively fresh water fish in the world. They have been found to reach a length of 15 ft/4m and can weigh up to 440lbs/200kg. And yes, for you smartypants out there, sturgeon are even larger than this, but they are not exclusively freshwater fish. Sturgeon spend most of their lives at sea, or in brackish water, and only swim into freshwater rivers to spawn. Another impressively large freshwater fish that was recently caught seems to have blown the Arapaima record out of the water - the Giant Mekong catfish (Pangasius gigas). This giant catfish inhabits the waters of the Mekong River in China, which flows southward into Southeast Asia. This fish is found as far south as Cambodia and Thailand where it is known as Pla Buk, which means simply "huge fish".The dimensions of these catfish are very impressive, with the largest supposedly growing up to 10ft/3m and weighing up to 660lbs/300kg.The European wels (Silurus glanis) is the largest of the catfish family, growing to 16.6ft/5 m, and weighing as much as 726lbs/330 kg.

Source(s): http://www.extremescience.com/BiggestFreshwaterFish.htm