It is improbable that you could put too much oxygen into the water. However, it is possible to create supersaturation in a pool which will harm your fish; however, this can only happen if oxygen is forced into the pool under pressure. This situation is possible in the unlikely event that the pipe leading into the pump develops a pinhole and sucks air into the pool. It is more likely that you have insufficient air in your pond. Air is added via the pump and aquatic plants that create oxygen. Pond owners may try to overcome problems of under-oxygenation by pumping more oxygen into the pool. This will add bubbles. Excess oxygen will evaporate.