Freshwater fish do not generally drink water, i.e. not for satisfying thirst. They imbibe water only as needed for respiration and release it through gills, which draw oxygen from that water. They also urinate in large quantities to prevent their tissues from becoming too watery.
Two terms known as Osmosis and Diffusion help answer the question. Osmosis: is a movement of liquid through a membrane from a lower to a higher concentration. Diffusion: is the movement of particles from an area of higher concentration to an area of lower of concentration.
In case of freshwater fish the liquids in their bodies are saltier than the water in their environment. Hence, Osmosis draws water into the fish’s body through its skin and gills. If freshwater fish did drink water they would blow up like balloons.
As against this, saltwater fish have to drink water because the water in their environment is saltier than the liquids in their bodies. Osmosis causes water from their body tissues to be released into their environment. Saltwater fish have to constantly drink large quantities of water to replace what they lose through Osmosis. Diffusion then releases the excess salts and minerals taken in and not needed. The salts and minerals exit through special cells at the base of their gills back into the saltwater. Saltwater fish urinate very little because of these processes. If saltwater fish didn’t drink the water they would shrivel up!