Can Fish Drown?
Fish can drown under certain conditions. It isn’t the water that keeps fish alive, but the oxygen in the water. Fish breathe oxygen, just as we do. The water contains oxygen that comes from the air or is given off by water plants. A fish takes the oxygen out of the water by means of its gills.
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.
When a fish is placed in a small tank of water, however, it may soon use up all the oxygen that is in the water. If this happens, the fish suffocates. This is why, if you keep pet fish, you must change the water frequently in order to give them a fresh supply of oxygen.