Today we want to refer to the correct way regulate body fluids in fish. To do this, we must start by defining osmosis and semi-permeable membranes. Osmolarity is the consequence of the concentration of solutes in a solution, that is, osmolarity is a consequence of the amount of salts that are dissolved in the water.
The semi-permeable membrane is the one that allows the passage of water (solvent) but not of the salts dissolved in it. The osmosis process is when two solutions that have different concentrations are separated by a semi-permeable membrane, the weaker solution flows through the membrane diluting the concentrated solution until both have an equal concentration.
La fish skin is a semi-permeable membrane that allows the transfer of water. The fish salt water and fish water Sweet have equal water levels, the same happens with the salt in your body, but they have very different mechanisms that allow maintaining the balance of body fluids and the salinity of the outside.
The concentration of salt inside a freshwater fish is stronger than that of the surrounding water, so the water is absorbed by the body through the gills and skin. Fish that live in fresh water do not have the need to drink water, they absorb it through their skin and their kidneys are responsible for maintaining the salts they need.
Instead, saltwater fish live with a stronger and more concentrated environment than they, that is why they are responsible for removing water from your body by osmosis. The water leaves the body to achieve equal concentrations with seawater. So that they do not get dehydrated they should drink sea water.