Freshwater fish live in rivers and lakes with very low salt levels, and saltwater fish live in seas and oceans that contain high levels of salt. The level of salinity in a fish's environment determines the type of fish it is. Common freshwater fish include trout, bass, salmon, and catfish. Most tropical, exotic and aquarium fish are freshwater fish.
According to Scientific American, salty environments draw a large amount of moisture from a fish's body tissues, causing it to constantly lose water through its gills and skin. In order to stay hydrated and survive, saltwater fish must consume very large quantities of saltwater. This causes them to produce very little urine, and they also secrete large amounts of salt through their gills.
A fish that lives in a freshwater environment has a normal balance of salt in its body since its environment does not constantly draw salt from its tissues. As a result, freshwater fish do not become dehydrated and produce large amounts of urine. Freshwater fish do not need to consume additional water through drinking.
There are very few species of fish that are able to live in both saltwater and freshwater environments. According to LiveScience, most fish are highly sensitive to salt-level chances that occur where they dwell.