Why can’t freshwater fish live in saltwater (and vice versa)?

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I was trying to read about why freshwater fish die in saltwater but it was still a little confusing to me.

In: Biology
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Take a human and put them on Mars. Now Sure there is oxygen on Mars, but it is a different type that will cause us to suffocate and die. Same goes for fish. Their gills can’t take it so they die

Speaking strictly of salt water or fresh water fish.. its more of an osmosis process..
Salt water fish cells have a high concentration of salt.. when put in fresh water, the cells get too much fresh water in them and takes the salt out, they get too bloated and die.

Fresh water fish cells have the completely opposite issue.. so when in salt water, their cells will get too concentrated with salt that they will get dehydrated and die.
Sometimes you see fresh water or salt water fish in brackish water ( the point where fresh water and salter water start mixing). They can hang out in the water for a while.. but eventually have to go back to the respective waters or die.
There are other fish such as salmon, steelhead, sturgeon and others and go back and forth between the too.. they are called Anadromous fishes. ( but most of them are born in fresh water, go out to the ocean, go back to fresh water to breed and die)

Source: my bf is a hard core Fishman. His fish knowledge is immense.

Edit: words, and more updated info from bf who loves talking about fishes..🤣🐟🐠

A living cell is designed to work at specific conditions: the chemicals that make up a cell perform best at certain temperatures, with certain concentrations of chemicals, etc. This also applies to how much salt is in the cell: living things need a certain amount of salt in them in order to survive. Even if the amount of salt inside the cell stays the same, the amount *outside* the cell also matters: if the fluid around the cell is saltier or less salty than the cell itself, the cell can dangerously shrink or swell as water flows through the cell’s membrane toward the saltier side. Living things have different ways of keeping the right amount of salt in their body, but they’re tuned for the environments they live in. For instance, humans’ urinary systems help keep the proper amount of salt in the body if they eat or drink most things, but they’re unable to get rid of enough salt to survive if a human drinks nothing but seawater. Similarly, most freshwater fish aren’t suited for keeping salt out of their body if they swim in sea water, and most salt water fish can’t keep enough salt *in* their bodies if they swim in fresh water.

It’s the same reason we humans can’t drink ocean water unlike river water. Our bodies aren’t made to deal with such high quantities of salt, same as fresh water fish. Well, someone has already explained it very well. I just thought of putting it in simpler terms: Salt water fish were born to be able to deal with the high amount of salt, while fresh water fish weren’t, so they’ll die if swapped places. Like how we humans would die from drinking salt water.

People write lot word when few word do trick:
Freshwater fish are less salty than saltwater, so water flows into them to dilute. Freshwater fish can’t take it, fish inflate and die