Air bubbles are caused by the water being saturated and no longer capable of holding the disolved air. The colder a liquid is, the greater it's capacity to hold gasses. That is why it is common to see air bubbles shortly after water changes. Generally, the water that we put into the tank is slightly cooler than that of the tank and the agitation of adding the water forces more air to become disolved. Once the water is warmed up, it becomes supersaturated and air bubbles form.