How do fish detect salt?
All animals need salt to survive, but unlike us, fish don't eat or drink to get their salt. They absorb salt directly from the environment.
Zebrafish live in freshwater, with low levels of salt. So, to survive they have to be very efficient at extracting salt from water. This means water with too much salt winds up being toxic for zebrafish.
I have been studying how zebrafish sense and then avoid salt water.