Can Fish Drown
They sure can! Fish require oxygen just like we do. And that oxygen is present in water in the form of dissolved oxygen gas.
Fish can drown if there is not enough oxygen in water, water can run out of oxygen if there are too many fishes and other creatures consuming the oxygen from water. Fish require a consistent intake of oxygen to survive. Their gills absorb oxygen from the water molecules as water passes through the gill membranes of fish.
The fish reacts harshly to low oxygen levels and this can be seen through slow and laborious breathing. And just like us as they consume oxygen and expel carbon dioxide. But at times in an enclosed water environment, like a fish tank, this gradually depletes the life-sustaining oxygen from the water.
The only place where new oxygen can enter the water is through the surface where water meets the air. And since this surface is limited compared to the total volume of water we see in aquariums, air pumps are used to bring oxygen-deficient water to the surface.
The bubbles that float to the top thereby creating a circulating effect and maintaining oxygen levels needed for the fish to survive. But even then if you put hundreds of fish in a small tank they will all still die even with aeration. Because the fish to water ratio is too great. The oxygen in the limited water is quickly consumed by the many numbers of fishes, leaving all of them breathless.
Similarly, the temperature of the water can also be a factor in fish getting enough oxygen. The amount of oxygen gas that gets absorbed in the water is inversely proportional to the temperature of the water. The hotter the water lesser will be the gas that will be in the water. So, if the water is too hot, sufficient air cannot dissolve in it and if there isn’t enough air, then they’ll drown. So technically a fish can drown if it is not able to obtain sufficient oxygen from the environment.