I don't know where else to ask this question, but I do know that with the flooding of AI, reliably Googling something is becoming much more difficult, especially if you are not already adept in that field of study. I have possibly learned a little bit, but I would like to ask someone who actually knows.

Why can't freshwater fish survive in saltwater, and the reverse? I was told it had something to do with the salt content of their cells, and the way they intake water, but I was curious on a deeper level. What biologically is happening to them? Does the freshwater fish become dehydrated as in their organs begin to dehydrate, and die? Or, is it happening on a purely cellular level, and we just see the damage after it's started to take hold? If I were to bring one back over to freshwater would their integument (idk if I used that word properly) need time to let the freshwater hydrate it too? Are there fish, or general waterbased life that can survive in both?