Why do saltwater fish need....well saltwater?

Question

I have a saltwater tank and a freshwater tank and I was asked by a child, "Why do the fish need to live in saltwater?" Well I didn't know the answer and I was curious myself, so if anyone knows please let me know?

Answer 1

because saltwater fish have a different overall osmotic pressure in thier bodies, they need the outside and the inside to be closer in pressure than freshwater fish do, which have a higher pressure inside thier body than the sourrounding water

saltwater fish shed salt, and freshwater fish can not
saltwater fish also drink water, and if they drank fresh water they would loose salt in thier body and have unusually low body pressure and burst...ewwww

Answer 2

Because they have always lived in saltwater. It is a different environment all together.