From Wikipedia, the free encyclopedia.
Fresh water (also freshwater or fresh-water) is water that contains only minimal quantities of dissolved salts, especially sodium chloride, thus distinguishing it from sea water or brackish water. All freshwater ultimately comes from precipitation of atmospheric water vapor, reaching inland lakes, rivers, and groundwater bodies directly, or after melting of snow or ice (see hydrologic cycle).
For fish it strongly matters how much dissolved sodium chloride the water they live in has. Most species cannot live in both fresh and salt water, though some species move between the two. Salt water fish have access to an abundance of salt, and try to get as much salt out of their body as possible, while trying to keep the water. Fresh water fish do the opposite: They have too much water, and too little salt.