Fish can drown in water if there is not enough oxygen in the water. Water can run out of oxygen if fish overpopulation uses up the oxygen. Fish require a consistent intake of oxygen to survive. Their gills absorb oxygen from water molecules as water passes through the gill membranes.
Excessive levels of oxygen intake are also lethal to fish, usually resulting in death. When a fish is taken out of the water, the gills take in too much oxygen from direct exposure to air. Fish need to maintain a balance between the intake of hydrogen and oxygen to survive.