I've already spoken about Zeb Hogan and his National Geographic powered Megafishes project, so advert to that post for more info about it.
Some days ago this "National Geographic Emerging Explorer" announced the catch, and release, of what is likely the world's largest known freshwater giant stingray and so probably the world's largest freshwater fish.

This beast, weighing an estimated 550 to 990 pounds (250 to 450 kilograms) was reeled in on January 28, 2009, in Thailand. The stingray's body measured 6.6 feet (2 meters) wide by 6.9 feet (2.1) meters long. The tail was missing. If it had been there, the ray's total length would have been between 14.8 and 16.4 feet (4.5 and 5 meters).

The giant stingray also known as freshwater whipray, can be found in a handful of rivers in Southeast Asia and northern Australia but much is still unknown about this mammoth ray species infact it was first described scientifically only twenty years ago, in 1989.
In the following picture you can see Zeb Hogan with the previous record holder:



More info on National Geographic news article. Check also the National Geographic info page about Giant Stingray