system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What about salt water fish prevents them from "breathing" fresh water?
gpt
It's not about breathing, it's about homeostasis. Saltwater fish have to constantly excrete salt from their bodies in order to maintain the electrolyte balance necessary for life. Put them in fresh water and they won't stop; they'll keep pumping salt out of their bodies until they die from lack of sodium.

Similarly, put a freshwater fish in salt water and they'll die from too much salt.