Saltwater fish are hypotonic to their surroundings. This means their blood has a higher water concentration than the surrounding sea water.
Due to the fact that there is more salt and less water outside their bodies, there is a tendency to take in salt and lose water.
To counter this, salt water fish drink a lot of the sea water and urinate very little. Special cells in the gills actively eliminate salt at the cost of extra energy and these fishes do not absorb any salt from the water they drink.
