Osmoregulation is the process of maintaining an internal concentration of salt and water in the body of fishes.

i. Freshwater fishes: 

The salt concentration inside the body of freshwater fishes is higher than their surrounding water. Due to this, water enters the body due to osmosis. 

If the flow of water into the body is not regulated. fishes would swell and get bigger. To compensate this, the kidneys produce a large amount of urine, 

Excretion of large amounts of urine regulates the level of water in the body hut results in the loss of salts. 

Thus, in order to maintain a sufficient salt level, special cells in the gills (chloride cells) take tip ions from the water, which are then directly transported into the blood.

ii. Marine fishes: 

Since the salt content in blood of marine fishes is much lower than that of seawater, they constantly tend to lose water and build up salt.

To replace the water loss, they continually need to drink seawater. 

Since their small kidney can only excrete a relatively small amount of urine, salt is additionally excreted through gills, where chloride cells work in reverse as in freshwater fishes.