Black Slime on Faucets: What It Is and How to Get Rid of It

Black gunk on faucet aerators, black flecks in the toilet tank, a dark film in the water heater: on well water this is almost always manganese and the bacteria that feed on it. Here is what it is and how to stop pruning it and kill the plant.

What is the black slime on my faucets?

Groundwater in much of the country carries dissolved manganese the way it carries iron. When it oxidizes it deposits as black-brown crud, and a family of bacteria makes a living speeding that oxidation up, building dark slime the same way iron bacteria build orange slime. The two often grow together, which is why some houses get orange stains and black aerator gunk from the same well. Check the toilet tank: dark slime on the flapper and walls means the colony is systemwide, not a faucet quirk.

Why does the black slime keep coming back?

The faucet is the tip of the vine. The colony coats the well casing, the pressure tank, and the pipe walls, and every wiped aerator regrows from upstream within weeks. Bleach-soaking the aerator kills what is on the aerator and nothing else, and adding bleach to the well runs into the same limits it always has: surface-burned biofilm and lost strength at well-water pH, covered in chlorine dioxide vs bleach.

Kill the colony at the source, not at the faucet.

Answer a few questions and we pull your well's dimensions from your state's official well record, then pack a chlorine dioxide kit to those exact numbers from $399. A contractor charges $1,500 to $3,000 for the same cleaning.

Find my well See what's in the kit

The fix

Shock the whole system, following how to shock a well, making sure the dose runs to every tap so the plumbing behind each faucet gets treated, and clean or replace the aerators while the treatment holds so the colony cannot reseed from them. Our kit runs chlorine dioxide at the manufacturer's published 20 ppm shock dose, which strips biofilm per SMT's data, and the residual test at the end tells you whether a heavy colony consumed the dose and earned a second round. If manganese tests high after the cleaning, that is when a manganese filter makes sense.

Straight answers

Is the black slime on my faucet dangerous?

The usual culprits, manganese deposits and manganese- or iron-feeding bacteria, are treated as nuisance organisms rather than pathogens. Two cautions: elevated manganese in drinking water is worth testing for on its own, and any slime colony can shelter other bacteria, so a slimy system plus a positive coliform test is a system to clean promptly.

Why does black gunk keep coming back on my aerators?

Because the aerator is just where the colony fruits. The growth lives back in the pipes, the water heater, and the well, and wiping the faucet prunes it without touching the plant. It returns until the system is treated at the source.

Is black slime the same as black mold?

Usually not. In well systems the black film is typically manganese oxide plus bacteria that metabolize manganese and iron. True molds want air and show up at the splash zones, not inside the pressurized plumbing. The inside-the-tank location is the giveaway that it is a water-system organism.

Does a water filter stop black slime?

A manganese-capable filter removes the dissolved food after the point it is installed, which helps. But a colony already established in the well and pipes keeps growing and keeps seeding everything downstream, including the filter itself, so clean first and filter second.

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