Black sludge in a toilet tank is usually mold from constant dampness, manganese or iron-feeding bacteria, or rubber breaking down from a flapper, float or seal. Rub a bit between your fingers. Soft, greasy smears point to a failing rubber part. Scrub the tank clean, then replace any part that leaves black on your hand.
What is the black sludge made of?
| What you see | Likely cause | Where it sits |
|---|---|---|
| Soft black smears that come off on your fingers | Rubber from a flapper, float or seal | Around the flapper, the fill valve, the bottom of the tank |
| Slimy black-brown film | Manganese or iron bacteria | Tank walls and floor, anywhere water stands |
| Fuzzy or speckled dark spots | Mold or mildew | Waterline, under the lid, the rim of the tank |
| Hard black crust or stain | Manganese minerals in the water | Waterline and the bowl |
### Rubber breaking down
Rubber and synthetic parts wear out. Braintree, Massachusetts, says black particles in toilet bowl water are likely from "the disintegration of the float in the toilet tank," and that the float "is especially likely to break down over time if you use an automatic toilet bowl cleaner in the tank." Kohler's guide agrees on the cleaner part: "Do not use in-tank cleaners in your toilet. Products containing chlorine (calcium hypochlorite) can seriously damage fittings in the tank." Fluidmaster adds that using such products voids its warranty on Fluidmaster parts.
Disinfectant can do the same thing. A Stanford University fact sheet, quoting the American Water Works Association, says chloramine "can cause the rubber or elastomer linings on older stainless steel braided flex connections and rubber flapper valves in toilet tanks to break down. This can cause soft, black flakes in the water." The same passage says the flakes sometimes look greasy and will "frequently stick and smear on surfaces." Phoenix does not use chloramine. Its Water Services FAQ says Phoenix's entire water supply is disinfected with chlorine. In a Phoenix home, age and drop-in tablets are the likelier culprits. Other Valley cities run their own systems, so check yours.
### Mineral slime from manganese and iron
Virginia Tech says bacteria that feed on iron and manganese "do not pose a health risk to humans, but do produce a red-brown (iron) or black-brown (manganese) slime in toilet tanks, water troughs, or other places where water stands." Penn State adds that manganese "often results in a dense black stain or solid." Both guides cover private household water. If you are on a city system, rubber and mold are usually the first things to rule out. Our page on black stains in a toilet bowl covers the bowl side of this.
### Mold in a damp tank
EPA says "The key to mold control is moisture control." A tank is wet all day, and a cold tank in a warm bathroom sweats. If the black growth is spotty and the lid or outside of the tank is damp, see why a toilet tank sweats.
How do you clean it out safely?
Keep it simple, and keep chemicals out of the tank.
- 1Shut the water off at the stop valve under the tank and flush to empty it.
- 2Scrub the tank walls and floor with a soft brush, warm water and dish soap. The CDC lists dish detergent as one way to clean household mold.
- 3Wipe the flapper seat and the flush valve opening with a cloth. Do not scrape the rubber.
- 4Rinse well, then turn the water back on.
- 5Check the parts as described below before you close the lid.
Never mix bleach with anything else. The CDC says "You should never mix bleach with ammonia or any other cleanser," which includes vinegar and toilet bowl cleaners. Bleach is also the thing makers warn about around tank parts, so skip it in the tank. If you want to use vinegar on mineral buildup, Fluidmaster recommends it for scale on the outside of its fill valve, followed by a warm soapy wash and a fresh water rinse. Whether to put baking soda or vinegar in a toilet tank explains why a full soak is a different matter.
When should you replace the flapper or fill valve?
Replace a part when it leaves black on your hand, feels soft or gummy, or no longer seals. Fluidmaster gives a leak test for the flapper: shut off the water to the toilet, leave the tank full, and mark the water level with a pencil. If the level drops to the lip of the flapper, "Fluidmaster recommends replacing the flapper."
For the fill valve, Fluidmaster says to look for "a bleached or faded type of discoloration," which "indicates age and wear that could compromise the function of the fill valve." It also says to replace the top cap assembly every 5 years, and to replace the water supply connector "if it is worn or over 5 years old." A black residue, a toilet that runs, or a fill valve that will not shut off are all reasons to act.
How to replace a toilet flapper and fill valve walks through the swap, and picking the right flapper helps you match the size. If tablets were the cause, what the makers say about cleaning tablets in the tank is worth a read before you reinstall anything.
What should you do next?
Clean the tank, replace any rubber part that smears, and stop using drop-in tablets. If the slime comes back within weeks, if you have black stains at every fixture, or if the black material shows up in your tap water, test the water rather than guessing. Our page on black specks in tap water shows how to trace those.
A leak at the base of the tank, a cracked tank or a fill valve you cannot reach is a job for a licensed plumber. HQ Plumbing & Air is licensed and insured and gives free estimates.