Why do fish that live in the ocean not retain salt?

Fish that live in the ocean do retain salt, through specialized adaptations. Most marine fish are osmoregulators, meaning they can control their internal salt and water balance. They actively excrete excess salt through specialized tissues such as gills and kidneys, while conserving water. However, some marine fish are osmoconformers, meaning their internal salt concentration matches the seawater, in which case they do not actively regulate salt, instead adjusting their internal water balance.