Why do freshwater fish produce large amounts of dilute urine?

Freshwater fish live in an environment where the surrounding water has a much lower concentration of salts than their internal body fluids. Due to osmosis, water is constantly absorbed into their bodies through permeable surfaces like their skin and gills.123

To maintain homeostasis—the balance of water and dissolved substances—freshwater fish must counteract this continuous influx of water. Their kidneys, which are often larger relative to their body weight than those of marine fish, function to excrete this excess water by producing large amounts of dilute urine.13

While this process effectively removes surplus water, it also leads to the loss of essential salts. To compensate for this, freshwater fish actively absorb salts from their environment through specialized cells in their gills, as they drink very little water and rely on these physiological mechanisms to survive in their hypotonic environment.13