Is it necessary to change the water in an aquarium periodically?

Like all other creatures fishes too need oxygen for breathing.But unlike other creatures fishes respire by their pair of gills.Their gills separate the oxygen from water and release out carbon dioxide.In lake or pond there is not much considerable change in the Oxygen level because: in a huge amount of water only negligible ratio of oxygen is used.Direct and indirect rainwater keeps the oxygen ratio stable.
          Water plants like Hydrilla and Vallisneria use the carbon  dioxide and deliver oxygen during their photosynthetic activity.But in the aquarium there is less amount of water and there are no natural plants to maintain the oxygen level.Another reason for changing the water is to keep the water clean from fish waste in aquarium.