The main reason for a bad-smelling reptile is an unclean tank. Leftover food, dead insects, urates, and other excrement all contribute to the bad smell. These objects lead to bacterial growth. These bacteria feed on unremoved organic debris and cause the smell. Spot cleaning and proper cleaning should get rid of the smell. Another cause of the bad smell is the substrate. Substrates such as sand can trap debris which can cause a bad smell. It is necessary to change the substrate within the enclosure regularly. I also advise against the use of sand because the sand easily absorbs waste such as excrement. When wet, sand encourages the growth of bacteria. Spot-cleaning sand can be tough. If you keep the enclosure clean but the beardie still smells bad, then you need to consider bathing it daily or once every two days. Another option is to use deodorizers to remove bad smells. Be sure to get one that is pet safe such as the Arm & Hammer Solid Gel Deodorizer or the Dynamic Element Reptile Tank Odor Eliminator Spray.