Salt/freshwater Fish Question

[ShinySideUp]

I have tropical fish and have no intention of becoming a salty but a question has popped into my head.
 
I can understand why freshwater fish would be unable to tolerate a salt environment, just as humans cannot tolerate too much salt, but why couldn't saltwater fish manage in fresh water? What does salt do for marine fish? Why would it's absence kil them?

 

[eaglesaquarium]

Osmotic pressure. They are designed to have a certain chemical composition inside their body, and when that chemical composition is fighting against the composition of the water, their body has to work harder. That would be akin to if humans were exposed to an atmosphere with a different chemical composition, like smaller amounts of oxygen and larger amounts of CO2. That would be a problem for people, just as the wrong salinity is a problem for fish.

 

[FishCat_16]

https/www.scientificamerican.com/article/why-do-some-fish-normally/
 
Can I post this?  It's kind of interesting and it even goes on to talk a little bit about anadromous fish as well.