Black Slime on Your Faucets? What It Is and How to Stop It

The first thing I do on a black-slime call is unscrew the kitchen faucet aerator. Nine times out of ten, out comes a smear of dark, greasy gunk that the homeowner has been scrubbing off for weeks. It looks like mold, and everyone assumes the worst. It usually is not mold at all. Here is what that black slime actually is, how we tell where it is coming from, and the fix that finally stops it.

Quick summary: Black slime on your faucets is usually manganese in your water, a mineral cousin of iron that turns black when it oxidizes, plus the harmless bacteria that feed on it. It is an aesthetic nuisance, not usually a health hazard at typical levels, and cleaning it never lasts because the manganese keeps flowing in. The only permanent fix is to remove the manganese and iron before they reach your fixtures. It is very common on Florida wells.

What causes black slime on faucets?

Black slime on faucets is usually caused by manganese in your water. Manganese occurs naturally in Florida groundwater, often paired with iron, and while it is dissolved and underground it is invisible. Once that water reaches a faucet and meets air, the manganese oxidizes into a dark, sticky residue, and bacteria that feed on it turn the film slimy. It collects wherever water sits or sprays, which is why aerators and toilet tanks show it first.

The University of Nebraska Extension notes that iron and manganese travel together in groundwater and behave the same way, staining and building up once exposed to air (water.unl.edu). Not every black film is your water, though. Sometimes the "slime" is biofilm feeding on a degrading rubber part, like an old toilet flapper. Browse more on well water and iron in our well water and iron guides.

What you notice	Likely cause
Black or dark film across many fixtures	Manganese, often with iron, in the water
Slime mostly in the toilet tank	Degrading rubber flapper plus biofilm
Reddish-brown slime	Iron bacteria
Pink-orange film in wet areas	Airborne bacteria (Serratia), not your water

Bottom line: Film that returns across several fixtures points to waterborne manganese; slime stuck in one toilet tank usually points to rubber parts.

Is it in my water or just one fixture?

To tell whether black slime is in your water or just one fixture, wipe two different aerators clean and watch what comes back. If the dark film reappears on several faucets within a few days, it is almost certainly waterborne manganese. If it is isolated to one toilet tank, replace the flapper and clean the tank first, since a degrading rubber part is the more likely culprit there.

When the film is clearly waterborne, a test settles it. A certified water test measures manganese, iron, and bacteria so the fix is matched to your actual numbers. The Florida Department of Health recommends testing private wells at least once a year through a state-certified lab (floridahealth.gov).

Is black slime from manganese dangerous?

Black slime from manganese is mainly an aesthetic problem, not a health hazard at the levels found in most homes. The EPA regulates manganese as a secondary (nuisance) contaminant with a guideline of 0.05 mg/L, the point where staining, a metallic taste, and dark buildup become common. There is no federal health-based limit for manganese at typical levels, but very high manganese has prompted health guidance, so if a test shows elevated levels, removing it is worthwhile as well as cleaner.

Can I clean black slime away myself?

You can clean black slime away yourself, but it will not stay gone if manganese is in your water. Soaking aerators and scrubbing fixtures removes what is there now, and replacing a worn toilet flapper fixes the problem when a rubber part was the source. Shock chlorination of the well knocks the bacteria back for a few weeks. None of it removes the manganese still flowing in, so the film keeps returning until you treat the water at the point of entry.

DIY fix	What it helps	How long it lasts
Clean or soak the aerators	Removes existing film	Days to weeks
Replace rubber flappers and gaskets	If the source is a rubber part	Long-term if that was it
Shock chlorination of the well	Knocks back the bacteria	Weeks to months
Cartridge filter	Particles only	Clogs quickly on manganese

Bottom line: DIY clears the symptom; only whole-house treatment removes the manganese that keeps the slime coming.

What is the permanent fix for manganese and iron slime?

The permanent fix is a whole-house system that oxidizes the manganese and iron and filters them out before they reach a tap. For most Florida wells, a backwashing air-injection (aeration) system converts the dissolved minerals into a filterable form and flushes them away on its own; higher levels or heavy bacteria may call for catalytic media or a light chlorine-injection setup. It is the same equipment family as our iron and sulfur removal systems, sized to your manganese and iron levels and your well's flow rate.

Cost depends on your manganese and iron levels, whether bacteria are present, your household size, and your flow rate, so ClearQuest does not quote a fixed price without testing your water first. Your free water test turns those variables into an exact, no-pressure quote, and most systems are installed in a single visit.

What does a black-slime service call actually look like?

Here is how a recent one went. The Whitfields are snowbirds who leave their Zephyrhills home empty from May through October, and they came back to black slime in every bathroom and a musty tank. They assumed the house had grown mold while it sat. When I pulled the aerators, the same dark film was on all of them, which already told me it was the water, not a fixture.

The free test confirmed elevated manganese with some iron, and the months of stagnant water had let bacteria bloom on it. The fix was a properly sized air-injection system set up for manganese and iron, installed at the point of entry in a single visit. New slime stopped forming immediately, and after one deep clean of the fixtures the black gunk was gone for good. For a home that sits empty half the year, treating the water at the source beat scrubbing every fall.

Why is black slime so common on Tampa Bay wells?

Black slime is common on Tampa Bay wells because manganese and iron ride together in the Floridan aquifer that feeds them. That means dark-film and black-slime calls are routine for us across Zephyrhills, Dade City, Land O' Lakes, Wesley Chapel, and rural Pasco. It is not a sign that anything is wrong with your well, just the local geology showing up at your fixtures.

If you are on a well and seeing black film, it is worth testing for manganese and iron even before it gets bad. See all the areas we serve across Pasco, Hillsborough, and Pinellas.

Frequently asked questions

Is black slime on faucets dangerous?

Black slime on faucets is generally an aesthetic nuisance, not a health hazard at the levels found in most homes. It usually comes from manganese, which the EPA regulates as a secondary contaminant at 0.05 mg/L, plus the harmless bacteria that feed on it. Very high manganese is still worth removing, so a test is the way to be sure.

Is the black slime mold or is it in my water?

Black slime on a well is more often manganese in the water than mold. A quick way to tell: if the film comes back on several faucets and aerators within days of cleaning, it is almost certainly waterborne manganese. If it is isolated to one toilet tank, it is more likely a degrading rubber flapper plus biofilm than your water.

Why do I only see black slime on some faucets?

You see black slime most where water sits or sprays and meets air, because that is where dissolved manganese oxidizes and bacteria collect. Aerators, the toilet tank, and low-use fixtures show it first. If it is truly limited to one fixture, the cause is usually that fixture's rubber parts rather than the water supply.

Can I get rid of black slime myself?

You can clean aerators, scrub fixtures, and replace rubber flappers, which helps if the source was a single part. But if manganese is in your water, the slime returns until it is removed whole-house. Shock chlorination knocks back the bacteria for a while, but they recolonize until the manganese is filtered out at the point of entry.

How much does it cost to remove manganese from well water?

The cost to remove manganese depends on your manganese and iron levels, whether bacteria are present, your household size, and your well's flow rate. ClearQuest does not quote a fixed price without testing your water. A free in-home water test turns those variables into an exact, no-obligation quote.

How fast does the black slime stop after treatment?

Once a properly sized whole-house system removes the manganese and iron before they reach your taps, new black slime stops forming right away. A professional system is typically installed in a single visit. Existing film still needs one last cleaning from the fixtures where it built up.

Done wiping black gunk off your faucets? Start with what is actually in your water - no guessing, no pressure.

Free Water Test & Consultation: in-home, multi-point test that measures your manganese and iron levels.

Sulfur & Iron Aeration System: chemical-free air-injection treatment that removes manganese and iron before they reach your fixtures.

Certified Lab Testing: accredited results if you need documentation or suspect bacteria are involved.

Call or text (813) 729-2125, or book your free water test online. No pressure, no obligation - just answers.

By Zach Brownell, ClearQuest Water Solutions - 10+ years treating Florida well water across Tampa Bay. Last updated July 2026.