Because fish breathe by filtering oxygen in water through gills, they can't absorb oxygen without water, and they will die if they lack oxygen.

In fact, it is called "Shenzi", which is a white striped fish. This kind of fish will soon die out of water. Fish absorb oxygen in water through four gills in gill cavity, and then discharge carbon dioxide into water one by one through gill cover. When fish leave water, water can't flow into gill cavity, gills will soon dry up, gills will stick together, and fish can't breathe with gills.

Without breathing, fish will suffocate and die. So fish spend their whole lives in the water, eating in the water, sleeping in the water and swimming freely in the water.