Purchased a home with a whirlpool tub. Never had one before. Used it off and on. Filled it up one night and started the whirlpool and black stuff came out, including tiny black pinworms. Disgusting !! I found your thread and purged the system with hot water and one and a half cups of Cascade Complete dishwasher gel. Ran it for twenty minutes. Then drained it and ran it with a tubfull of plain hot water for twenty minutes. Did this three times. It seems to be okay now. What in the heck were those tiny little black pinworms ???