Oxygen concentration plays a role in the fish by helping them breathe. Without enough oxygen, the fish would die. However, if there is too much oxygen it could be a sign of too much algae, which hurts fish in the long run, because too much unbalance in an ecosystem is actually really harmful, in the way it will start polluting the water by blocking out sunlight and warmth and cause too many disruptions in the natural order. It can also cause diseases.