How do freshwater fish replace salts lost to the surrounding water?

Freshwater fish live in environments with a lower salt concentration than their internal body fluids, causing them to constantly lose salts to the surrounding water through diffusion while simultaneously absorbing excess water via osmosis.12

To maintain homeostasis, freshwater fish replace lost salts through the following mechanisms:

• Active Absorption: Special cells located in the gills and the skin inside the mouth actively pump salts from the surrounding water into the body, moving these ions against the diffusion gradient.12
• Dietary Intake: They replenish a portion of their required ions by consuming food.12

Because they are constantly absorbing water through their permeable surfaces, these fish also produce large quantities of very dilute urine to excrete the excess water and maintain their internal salt balance.123