Why does my hot water heater smell like rotten eggs?

Hard water deposits building in the water tank and on the elements. Drain the water heater and flush it out and change the elements . Sounds hard but it's really not , It will solve the problem for a few more years until it builds up again. If you don't want to keep going threw this you could install a filter on the incoming water line to the water heater. It will keep this from ever happening again and the filter is only about $30 at wal mart or lowes.