Because their blood chemistry is designed to be in homeostasis with a saline medium. In fresh water, water will enter the fishes tissues and kill it.
However some fish (eels and salmon) have evolved to live in both environments.
Salt water
Fesh
fresh water fish
Saltwater fish don't live in freshwater.
Saltwater fish require salt water, and fresh water fish need fresh water, you can't mix them, or the fish will die. You need to look at each individual fishes needs as to what temp they need their water to be. There are some saltwater fish that can survive in fresh water, but it is not healthy for them.
saltwater fish need to breath in saltwater and not fresh water. just like we need to breath in oxygen and not carbon dioxide.
No, a brook is a small stream of fresh water. So the brook trout is a fresh water fish, not a salt water fish.
they drown
Some Can And Some Can't.
Saltwater
It depends on the type of fish. If it is a saltwater fish than it lives in salt water, but if it is a fresh water fish than it lives in freshwater.
I am very sure they are freshwater fish, not saltwater.