Its look like empty from outside.. who knows about inside?

Mega freshwater fish worldwide

Human beings have always hunted top predators from sharks in the oceans to lions, tigers and bears on land. It is no exception for the biggest fishes in rivers. The large mega-fish species used to threaten the life of people in general and fishermen in spectacular. However, these have become rare worldwide due to over-fishing, habitat destruction and pollution.

 An adult arapaima fish, known as the pirarucu in Brazil or the paiche in Peru, is fed at the aquarium in Manaus, Brazil and becomes one of the largest freshwater fish worldwide. Some reach lengths of more than 10 feet (3 meters) and weigh upward of 400 pounds (180 kilograms)

Visitors saw a preserved paddlefish at the Yangtze River Fishery Research Institute in Jingzhou, China

 This dangerous species lives in China's Yangtze River and ranks the largest freshwater fish in the world. Unfortunately, overfishing and dam construction have decimated paddlefish populations

  pair of Chinese sturgeons swims in the Beijing City Aquarium. Capable of weighing half a ton and growing to 16 feet (5 meters) long, this freshwater leviathan is one of the largest sturgeon species on Earth

 A giant barb was caught on the Tonle Sap River near Phnom Penh, Cambodia. There is evidence that the species can reach sizes of 10 feet (3 meters) long and 660 pounds (300 kilograms)

 A Cambodian man came face-to-face with a giant barb on the Tonle Sap River near Phnom Penh. Fishermen along the Tonle Sap catch five to ten adult giant barbs each year, making the region one of the last places on Earth to see these impressive fish at their largest

 A newborn giant stingray was caught by local fishermen on the Banpakong River near Bangkok, Tha
iland. 

 The mother stingray measured 14 feet (4 meters) long and weighed from 200 to 300 pounds (90 to 140 kilograms). Scientists hope the presence of a baby and a pregnant mother means the river is a nursery for these freshwater giants

 Mekong giant catfish is one of the largest fish in the world. This fish this can reach 10 feet (3 meters) long and weigh up to 650 pounds (300 kilograms)

 A Cambodian man observed a Mekong giant catfish on the Tonle Sap River

Two boys carried giant river catfish in Cambodia on their shoulder

The giant Eurasian trout, also known as a taimen, is famous for its voracious appetite. This 5-foot (1.5-meter) specimen was found on the banks of the Eg-Uur River in northern Mongolia after killing a 3-foot (1-meter) victim of the same species

reference http://www.liveaquaria.com/product/aquarium-fish-supplies.cfm?c=830