Freshwater fish - live exclusively in fresh water - rivers, lakes, reservoirs and are not found in salty and brackish waters. They breed mainly in or near the same places where they constantly fatten up. They include all river and lake species (tench, crucian carp, pike, perch, etc., totaling about 8.3 thousand species).

The importance of salinity for fish is expressed mainly in the fact that it affects the osmotic pressure. It is the difference in osmotic pressure of water of different salinity is the main reason that prevents the transition of fish from the sea to fresh water and back. Only a relatively few, predominantly transient, fish have the ability to adapt to different water salinity in certain periods of life.

Freshwater fish differ physiologically from saltwater fish in several respects. Their gills must be able to diffuse dissolved gases while retaining salts in body fluids. Their scales must reduce diffusion of water through the skin: freshwater fish that lose too many scales die. They must also have well-developed kidneys that excrete large amounts of highly dilute urine.