Final answer:
Freshwater fish actively pump out ions via their gills and excrete dilute urine to maintain appropriate solute concentrations in their bodies.
Step-by-step explanation:
Freshwater fish live in an environment that is hypotonic to their cells. As a result, they absorb water through their skin and gills. To maintain the appropriate solute concentrations in their bodies, freshwater fish actively pump out ions via their gills and excrete dilute urine to get rid of excess water.