From Microcosm Aquarium Explorer
Freshwater: A Definition
Freshwater ecosystems providing organisms of interest to aquarists include streams, ponds, lakes, rivers, swamps, wetlands, and flooded forests.
Freshwater is typically defined as having less than 1% salt content.
Salinity of Different Ecosystems
Measured as total dissolved salts in parts per thousand (ppt):
- Freshwater < 0.5 ppt
- Brackishwater 0.5 - 35 ppt
- Saltwater 35 - 50 ppt
- Brine > 50 ppt
- The largest lake on Earth, with about 20% of the total water on the planet, is Lake Baikal in central Asia. It is more than a mile deep--5,315 ft (1,620 m).
- The largest river on Earth, as measured by total water volume it discharges into the sea on the Brazilian coast is the Amazon. This flow has been estimated at 7 million cubic of water per second. The Nile River is usually cited as the longest river, at 4,049 miles (6,515 km), but new evidence suggests the Amazon may actually be longer.
- The smallest known freshwater fish (and smallest of all vertebrates) is the recently discovered Paedocypris progenetica, a tiny minnow (Cyprinidae) from the peat bogs of Sumatra. Adults are about the size of a mosquito, 7.9–10 mm (.3–.39 in.).
- The largest freshwater fish species is believed to be the Giant Mekong Catfish (Pangasius gigas) of Southeast Asia. A record specimen caught in 2005 weighed in at 646 lbs. (293 kg) and 9 ft. (2.74 m) long. the longest freshwater species recorded, at more than 15 ft (4.6 m) is the South American Pirarucu (Arapaima gigas).