Is the small white cover on the top or bottom of the water heater? The bottom may be the sewage outlet of the water heater. The water tank of the water heater is changed once a year or so, and the magnesium rod of the water heater is replaced at the same time. Magnesium rod is used to remove the metal in the water heater to prevent the water tank from being corroded and leaking. In order to prolong the life of the water heater, the water heater must be replaced regularly according to the instructions.