Can a fish drown? Yes! A fish would drown in water that is not sufficiently oxygenated.
Actually the question is not a silly as it sounds and does not only occur when there is not enough oxygen in the water.
Under the right conditions (or in this case, the wrong conditions) a fish can actually drown just as surely as a human can. Drowning is caused by suffocation in a body of water, it is accurate to say a fish that is starved of oxygen while in the water has in fact drowned.
Fish breathe by taking in oxygen from the water and if the body of water they are living has a depleted oxygen supply, then the fish must either move to a new, oxygen rich area, or drown. During summer months in particular, the deep lakes’ oxygen supply can easily become depleted due to animal respiration and decay of vegetation. This causes large groups of fish to die, as they very often refuse to move out of the oxygenless region.
There are even some species of fish that have developed such complex respiratory systems that they can only breath outside of the water. Fish, such as the Climbing Perch Fish, can get out of a body of water and survive on land, but many need to breathe our air as well as gaining oxygen in the water. If these fish are forced to stay under water, they will not gain the necessary oxygen they need to survive, resulting in the fish drowning. The Climbing Perch Fish have a respiratory system that is depended on our oxygen rich air and the fish will drown if forced to stay under water for to long, no matter how much oxygen is in the water.
(Answered by Wiki)