Freshwater vs Saltwater Fish: Key Differences Explained

41% of all fish species live in freshwater despite it being only 0.01% of Earth water. Saltwater fish drink constantly and excrete salt; freshwater fish absorb water and excrete it. Salmon and eels transition between both.

Freshwater fish tend to have smaller ranges and higher extinction risk. The Amazon has 3,000+ freshwater species. Osmoregulation is the key physiological challenge — fish must maintain internal salt balance in very different environments..