Since freshwater fish are surrounded by a hypotonic environment, their problem is quite different. They must get rid of the fresh water that is constantly moving into their bodies. To drink water would be counterproductive, and so they never drink. Instead, the little salt that is present in the water around them is drawn in across the gills. This process also requires energy and involves the participation of ATP-splitting enzymes in glands within the gill.
Why do many marine fish drink seawater, whereas freshwater fish do not drink at all?
Since freshwater fish are surrounded by a hypotonic environment, their problem is quite different. They must get rid of the fresh water that is constantly moving into their bodies. To drink water would be counterproductive, and so they never drink. Instead, the little salt that is present in the water around them is drawn in across the gills. This process also requires energy and involves the participation of ATP-splitting enzymes in glands within the gill.