Black Mold in Toilet Bowl & Tank-Causes and How to Get Rid

If you have seen dark stuff, spots, residue, or ring in your toilet bowl, and behind, underneath, or inside your tank, it is probably mold or mildew. The discoloration could change over time to different colors such as green or orange but it is mostly black.

Black mold in your bathroom is brought about by dampness, stagnant water, and poor ventilation. Although it is unsightly to see and can possibly impose a bad image on your levels of hygiene, toilet mold is quite a common problem and getting rid of it is pretty easy and quick.

Following is a detailed discussion about the causes, how to deal with the mold including preventive measures and answers to commonly asked queries.

Black Mold in Toilet- What it is & Causes

Black, sometimes orange or green, discoloration is a fungus that grows vigorously in a moist, dark, and warm habitat. Bathrooms provide the perfect conditions for fungus.

The mold mostly inhabits toilet bowls/tanks, showerheads, shower doors or curtains, sink drains, bathtubs, tiles and grout.


Unlike other toilet discolorations such as the brown stains that are associated with problems to do with water quality, toilet mold is mostly linked to moisture. The following are some of the common most causes the mold in your bathroom

  • Infrequent use of your toilet is one of the most obvious causes of mildew. This provides, warm and moist conditions quite warm which sets up a breeding ground for mold. In addition, reduced flushing cuts off chlorination that helps destroy the mold.
  • Stagnant water – the tank is a neglected area of the toilet. As so, the area never gets cleaned out leaving it safe for the mold to grow, leaving them to feed on algae and other nutrients found in stagnant water.
  • Darkness – yes, when the lid is added to the tank, the mold tends it thrive more since it’s an opportunistic fungus that doesn’t require chlorophyll acquired from sunlight.
  • Mineral deposits – surfaces such as the tank’s walls and hard water deposits help supply mold with a nutrient source. The more water stays put in the tank, the more deposits shall gather in the tank causing mold to infest more and more. Sometimes the condition is worsened by the type of water; hard water contains more minerals than soft water.
  • Worn-out washer – this is prevalent if you see there’s mold underneath the tank. The purpose of the washer is to seal water from passing through. So if the area has mold, it means there’s some form of leakage and the washer no longer does its job adequately.
  • Human waste in the toilet can encourage mold growth. Mold is said to love sugar, more sugar in your excrete can lead to more mold in the bowl.
  • Water supply from pipes can also be a source of black mold in your bathroom. Rust on the pipes can also cause condensation hence breeding grounds for the fungus. This is usually the cause of recurring black residue in especially in your tank.
  • Cracks in the ceramic bowl, tank, and other bathroom fixtures can cause mold. These cracks provide the fungus a conducive environment to stick, grow and flourish.
Black mold below tank
Mold below tank

How to Remove Black Mold/ Spots in Toilet Bowl

In the bowl, black stains are usually found under the rim, below the waterline or at the base/bottom.

Dealing with Black mold toilet bowl
A toilet bowl with black mold

Mold forming at the water line can be an indication that you rarely use the toilet, your tank is infested with the mold and it’s time for a thorough clean up.

When mold grows below the water line and more around the base of the toilet bowl, it could mean you have a broken water seal that needs immediate attention from the plumber. The broken seal means the area is experiencing a leak underneath or from the water inlet.

A leak then means a damp area and a conducive environment for the growth and development of mold. The plumber will need to replace the wax ring to stop the leak then you can clean the area off the mold and that’s all.

Following are the steps to clean mold in the bowl

  1. Put together all the essential items you will need like protective clothing – gloves, a breathing mask. A bucket, rags, some vinegar, borax, baking soda, and a cup of bleach for a thorough clean.
  2. Open the door and windows for the bathroom area to ensure the room is properly ventilated.
  3. Cover your hands gloves and face with the mask
  4. Start off by flushing the toilet for a clear view of the mold spots.
  5. Rinse out all the water from inside the bowl to view the water like and the base of the toilet.
  6. Add in the drain cleaner first at the far end of the bowl so it goes down the drain.
  7. Sprinkle some baking soda or borax on the bowl with added attention to the waterline.
  8. Continuously spray vinegar over the baking soda and scrub gently with a hand brush, old toothbrush or a nail brush. Clean it for some time to loosen the fungus.
  9. Leave for about half an hour to an hour.
  10. Come back, scrub again, then flush the toilet for a clean rinse.
  11. Add in some bleach to the water and leave unattended for another hour.
  12. After that, pour in sparkling hot water to the bowl to activate the bleach further, then flush the toilet.
  13. If unsatisfied with the results, repeat the procedure several times. Clean up your cleaning items when done.

This procedure is only best at the first instance of sighting the mold infestation. After that, regular cleaning can prevent the need for aggressive cleaning.

Recommended Reading: How to Clean Mineral Deposits, Dirt, and Black Stuff under Toilet Bowl Rim

How to Remove Black Mold in the Water Tank

Once you’ve spotted the mold from the water tank, you need to clean out the area thoroughly to get rid of the mold. There are several ways you can do that, including simple DIY ingredients such as vinegar or lemon juice mixed with baking soda.

Materials Needed

  • Soft hand brush
  • Vinegar
  • Bleach


  1. Flush out water from the tank while turning off the water inlet. Ensure the tank remains empty for some time.
  2. Pour in vinegar into the water tank by spraying. Make sure all the corners are attended to. Leave on for half an hour.
  3. Scrub the toilet tank gently to scrape off any mold that may not have come out yet.
  4. Turn on the water outlet and let the water fill up again. Add some bleach and let sit for another half an hour.
  5. Flush the water and check to see if the mold has cleared. If not, repeat until you achieve the results.

It is vital to note that bleach or vinegar can affect some parts of the tank hence one needs to be prudent about how they add the bleach or vinegar to the tank.

How to Remove Mold Behind Toilet Tank

When water drips at the back of the toilet tank, mold can grow there as well. Luckily, removing mold from behind the tank is simpler. Here’s how:

Black mold behing tank of toilet
Mold behind tank

Materials Needed

  • Bucket or container
  • Rags
  • Soft hand brush
  • Vinegar
  • Baking soda
  • Disinfectant/ Bleach


  1. Wipe down the back of the toilet tank to get a clear glimpse of where the mold is.
  2. Make a paste in a dish using baking soda and vinegar. Ensure the paste is thick enough to stick behind the toilet tank.
  3. Apply the paste over the mold-infested areas ensuring to cover each and every section of the back area.
  4. You can scrub gently with the hand brush to make sure the paste gets in between the grout areas of the tank.
  5. Leave it on for half an hour.
  6. Once back, scrub the back of the toilet gently to scrape off the mold that may not have come out yet.
  7. Wipe down with a rag to see any areas that may need further scrubbing.  Continue until you have your results.
  8. Spray clean water on the area for a clean finish. You can also add some bleach to disinfect.
  9. If the mold spots or stains haven’t cleared, repeat until you achieve the results you want.

Why is there Black Stuff in Toilet after Flushing?

If you notice black stuff after flushing your toilet it means that either the pipes that supply water to your house have some rust or leaks. The leaks and rust harbor black mold that is then transferred to your toilet tank.

At the same time, black stuff could indicate an issue with the water itself. The water may be dirty, with mineral deposits or some unidentified residue that is black.

Is Toilet Mold Dangerous-Can it make you Sick?

The black mold growing in the tank, behind it, and under the toilet bowl can be a hazard to anyone, healthy or unhealthy. According to health experts, exposure to mold can lead to a stuffy nose, wheezing, red or itchy eyes and skin, hay fever and shortness of breath.

It is worse especially for individuals who are allergic to spores. It is recommended that you have safety measures put in place so as not to encourage its growth. You could put on a respirator so that the spores from the mold don’t trigger allergic reactions as you clean.

Black Mold in Toilet Bowl Diabetes

There’s no direct link between mold and diabetes. It, therefore, means mold doesn’t cause diabetes. However black mold could highlight certain symptoms of diabetes in your body.

Well, mold is known to thrive in sugar, when one is suffering from diabetes, the body doesn’t adequately hold on to enough glucose but rather releases it through urine.

Hence, when you notice a reoccurrence of mold in your toilet with all other factors constant, it could mean your urine contains too much sugar meaning you may be suffering from diabetes even without your knowledge.

The link then appears in that mold infestation could be a major indication of your blood sugar levels in sweat and urine, warranting the need for a checkup.

Prevention Measures

To avoid any further infestations of mold, the following simple preventive measures can help.

  • Ventilate the bathroom during and after you take a shower. This is because the area is damp, which then inhibits the growth of mold. Opening the windows or using a fan keeps the area cool.
  • Use dehumidifiers when you can open the window such as at night or maybe during a cold season.
  • Always clean up after using the toilet. Flushing down your waste even after using it for a short call.
  • Regularly clean the toilet using scrubs and chlorinated cleaners.
  • Fix any bathroom leaks as soon as they start.

Further Reading

One Comment

  1. Hello my friend! I want to say that this post is amazing, nice written and include almost all vital infos. I would like to see more posts like this. Mariele Isadore Donny

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