Evaporating Aquarium Water

This other guy worked at an aquarium store. Late one night he was cleaning up when the phone rang. He picked it up and started talking to a frantic lady. She explained her son had just left for college and had left her with an aquarium. The fish were all still healthy, and there was really nothing wrong with the tank, but the water level was mysteriously dropping. Her question, "Is it natural for water to evaporate?"