Black buildup in a home toilet tank is most common in areas where water supplies have a large concentration of iron, manganese and calcium. Iron or manganese bacteria flourish in these areas because they feed the iron or manganese in the water, which produces that black-brown slime you see accumulating in your toilet tank. Vinegar kills mold, germs and bacteria as well as removes stains and is effective when it comes to cleaning the black buildup in a home toilet tank.
Pour 2 cups of vinegar right into your toilet tank, and allow it to sit for 10 minutes.
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Scrub the black buildup away with a scrub brush. Depending on how long the black buildup has been there, this step may require vigorous scrubbing.
Flush the toilet, and allow the tank to fill up with water.
Assess the amount of black buildup left in the toilet tank. It should be completely gone, but if you notice any remaining, continue to scrub at it.
Repeat steps 1 and 2 if there is still a considerable amount of black buildup noticeable. The longer the black buildup sits in a home toilet tank, the more difficult it is to remove.