Freshwater fish never drink water, but saltwater fish drink constantly
Whether a fish drinks depends entirely on osmosis. Because a freshwater fish's internal fluids are saltier than the river around it, water naturally diffuses into its tissues through its skin and gills. Drinking would cause it to swell up, so it never drinks and urinates constantly. Saltwater fish face the opposite danger: the salty ocean leaches moisture out of them, forcing them to gulp seawater continuously and excrete the excess salt.
The Invisible Tug-of-War Beneath the Surface
A fish swimming through water lives in an environment that constantly pulls at its internal fluid balance. Even though these animals spend their entire lives immersed in liquid, their survival depends on managing a relentless physical process that threatens either to flood their cells or drain them dry. This invisible pressure is governed by osmosis, the natural tendency of water molecules to move across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration.
Because fish require continuous gas exchange to breathe, they cannot simply cover their bodies in a completely waterproof barrier. The delicate membranes of their gills and the linings of their mouths must remain thin and permeable so oxygen can diffuse into the bloodstream and carbon dioxide can diffuse out. This essential biological exposure creates an open doorway for water and dissolved minerals to interact, forcing every aquatic creature to negotiate an ongoing physical tug-of-war with its surroundings.
Freshwater Fish and the Threat of Overhydration
In lakes, ponds, and rivers, the water contains very little dissolved salt. The internal tissues, blood, and fluids of a freshwater bony fish, by contrast, are loaded with vital ions, proteins, and minerals. Because the fish's internal environment is significantly saltier than the river around it, the fish is hypertonic to its habitat. Under the laws of osmosis, surrounding freshwater is drawn steadily inward across the permeable membranes of its gills and oral surfaces, seeping directly into its circulatory system.
Under these conditions, swallowing water would be catastrophic. Gulping river water would introduce even more moisture into the digestive tract, rapidly diluting essential electrolytes and causing bodily cells to swell and rupture. For this reason, freshwater fish do not drink water intentionally. Any moisture entering their digestive tract is merely incidental, swallowed along with food. Their primary physiological struggle is not finding moisture, but ridding themselves of the vast surplus entering through their respiratory surfaces.