There are black, greasy particles in the bottom of our gas hot water heater. We drained it and they redeveloped. The particles are soft and smear when touched. These particles come out of the hot water taps but not the cold water taps. The house was vacant for several months when we purchased it. It was in estate. We know not much if any water was drawn. The hot water temperature was set on vacation. Does this mean the tank needs replaced or is it some part.