Watching Amazon river fishes: Here is a glimpse into the rich and vibrant underwater life of the Amazon. When the large, serpent-like arapaimas swim past the camera, you will get a close-up look at one of the largest freshwater fish in the world. The ones you can see here range from five to six6 feet in length, but may reach up to ten feet and weigh 300 pounds. Red-tailed catfish, arowanas, black pacus, and Poecilia reticulata (guppy) share this 55,000-gallon aquarium below a living tropical forest.