Of course fish can also feel thirsty because every living thing needs water. There is a difference between fish that live in fresh water and fish that live in salt water. Osmoregulation in freshwater fish, these fish tend to absorb water from the environment by osmosis. Freshwater gills actively incorporate salt from the environment into the body.

The kidneys will pump out excess water as urine. The kidneys have large amounts of glomeruli with large diameters. This is intended to be more able to withstand body salts so as not to come out and at the same time pump as much urine as possible. When the fluid from the malpighi body enters the kidney tubule, glucose is reabsorbed in the proximal tubule and salts are reabsorbed in the distal tubule. Kidney tubule wall is impermiable to water. The resulting urine contains high water concentrations.