Freshwater fish are fish that spend some or the majority of their lives in freshwater ,eg brooks and lakes, with a salinity of less than 0.05%. These environments vary from sea conditions in a number of ways, the most blatant being the difference in levels of salinity. To survive clean water, the fish need a variety of physical adaptions so as to keep the ion density of their bodies balanced.

 41% of all known species of fish are found in freshwater. This is basically because of the quick speciation the scattered habitats make practical. When handling pools and lakes, one might use identical fundamental models of speciation as when studying island biogeography. Fresh water fish differ physiologically from salt water fish in a few respects. Their gills must be well placed to diffuse melted gasses while keeping the salts in the body liquids within. Their scales reduce water diffusion thru the skin : freshwater fish that have lost too many scales will die.

They also have well developed kidneys to reclaim salts from body liquids before excretion. Many species of fish do reproduce in freshwater, but spend the majority of their adult lives in the ocean. These are known as anadromous fish, and include, as an example, salmon, trout and three-spined stickleback.

 Some other types of fish are, to the contrary, born in salt water, but live the majority of or parts of their adult lives in clean water ; for example the eels. Species migrating between sea and fresh waters need adaptions for both environments ; when in salt water they have to keep the bodily salt concentration on a level lower than the environment, and vice versa. Many species resolve that difficulty by associating different habitats with different stages of life. Both eels, anadromous salmoniform fish and the sea lamprey have different tolerations in salinity in different stages of their lives.