system
Given the following post, respond as if you're commenting on Reddit:
human
What would happen to a salt-water fish if you threw it into freshwater?

Alternatively, can you season a freshwater fish by throwing it into saltwater to die?

Ignoring ethical issues and all.
gpt
Most fish species would die from osmotic shock, however evolution has equipped some to survive this transition.  Salmon transit from fresh to salt and back again, and eels from salt to fresh,  However many more species, if gradually equilibrated, can live quite well in the other environment.  Guppies come to mind as a fresh-water species that can be adapted to seawater.

In nature there are many partly salty waters, called brackish, such as the central African lakes that support their own half-way ecology.