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What is the Black Stuff Under the Toilet Bowl Rim?

What is the Black Stuff Under the Toilet Bowl Rim?

Toilets can be a cesspool of germs and disgust if left unattended. While flushing after every use and occasional scrubbing with a cleaner of your choice should be enough, there comes a time when you have to up the ante and do some thorough cleaning.

This could be after getting some yellow urine stains, stubborn hard water stains, or that mysterious black gunk under the toilet rim.

What Causes Black Sediment Under the Toilet Rim?

A couple of things could lead to these unsightly black rings under your toilet bowl. 

Mold, Mildew, or Algae

In most cases, the black stains under your toilet rim are fungus, mold, and mildew. A toilet bowl has a set of nozzles in the rim that dispense water every time you flush.

As time goes by, sediments from your water accumulate around these nozzles making the area around them less slippery than it used to be. Hard water deposits and small pieces of dirt accumulate here, creating the perfect foothold for mold and mildew.

The area under the rim won’t clean as well as before, and the mold or mildew will get the nutrients it needs to thrive. Since the area probably stays moist for long, the colony will have all it needs to thrive.

ProTip: If your cistern is dirty and has algae or mold, it will be easier to transfer to your toilet bowl’s rim.


Under very rare conditions, sewer microorganisms can thrive here as soon as the mold and mildew take hold. However, you will have to neglect your toilet a lot for this to happen.

Bacteria grows in moist places and is present in fecal matter. It can quickly establish under your toilet rim since it is always moist and easy to ignore when washing the rest of the toilet.

Mineral Deposits

Finally, the black, red, brown, gray, or yellow rings under your toilet bowl could be mineral deposits from your water. The color varies depending on which mineral is prevalent in your tap water.

If you use processed municipal water, it will take ages for such residue to form. However, if you use well water or your water supply is hard, the calcification and deposition of minerals will be faster and easier to note.

The minerals will encourage mold, algae, and bacteria colonies to make an even bigger mess.

How to Get Rid of the Black Stains Under My Toilet Rim

Getting rid of the black mold under your toilet rim shouldn’t be hard. As it turns out, mold and algae are easy to kill using traditional toilet cleaners and even some makeshift cleaning solutions.

All you have to do is pay attention when cleaning the toilet and focus on cleaning under the toilet rim. 

Start By Cleaning the Toilet With Your Favorite Toilet Cleaner

The first step is disinfecting your toilet by cleaning it thoroughly with your favorite toilet cleaner and a brush.

Best Product for Cleaning Under Toilet Rim

If you have been using soap or just flushing the toilet with plain water, you have to step up and invest in a dedicated cleaning detergent. You could try out the following if you don’t have anything.

Here is what to do

  1. Apply the toilet cleaner around the top of the toilet rim. Ensure you apply a heavy coat so that it starts to run down the toilet bowl
  2. Let it sit for around 10 minutes to take effect
  3. Using your toilet cleaning brush or wand, scrub the toilet rim thoroughly. Keep scrubbing for around three or so minutes to ensure you get everything done
  4. Wash the rest of the toilet bowl
  5. Flush the toilet twice to rinse it

After rinsing the toilet, most or all of the black mold should now be gone. However, it will come back if you don’t take measures to prevent reinfection.

How to Prevent Black Mold From Growing Under Your Toilet Rim

Cleaning the black mold should be easy and fast. Don’t rest after a cleaning session. Here are some extra steps you should take to ensure you don’t face the same problem again.

Get Rid of Mineral Deposits

With the toilet clean, you can now focus on removing any calcification or mineral deposits under the toilet rim. The technique will vary depending on the deposits type. However, in most cases, they are hard water deposits.

You can remove these using vinegar.

  • Add a cup of vinegar into a spray bottle and spray it onto the toilet rim. Keep spraying until it is fully saturated.
  • Let it sit for around five minutes.
  • Sprinkle a half cup of baking soda onto vinegar and let it fizz until it stops
  • Flush the toilet and use a brush to scrub away 
  • Flush again twice to get rid of all the residue

Cleaning under the toilet rim with vinegar is a great way to get rid of hard water deposits. However, it shouldn’t be a substitute for bleach or a dedicated toilet cleaner since vinegar isn’t as good a disinfectant as these can be.

Clean and Disinfect the Cistern

Toilet cisterns (the reservoir holding flushing water) are often neglected. Most people will clean the outside and ignore the inside completely.

As a result, dust, mineral deposits, mold, and algae take root, making the place disgusting. Lift the lid on your cistern, and you will be surprised how dirty it looks.

You can remedy this by using a quick clean and rinse. Since it is not in direct contact with the toilet bowl, you can use plain soap to clean. I still go for the toilet cleaning detergent, to be sure.

  • Put on some protective gloves.
  • If there are any visible solids or large residue, pick them up and remove them from the cistern.
  • Get a new toilet washing brush or brush on a handle
  • Add some liquid soap or the toilet cleaning solution into the cistern (never both)
  • Use the brush to scrub around and get rid of as much of the dirt as possible
  • Use your hand again to remove big chunks of dirt knocked loose when scrubbing. These could block the flushing mechanism
  • Flush the toilet to empty the cistern and allow in a fresh batch of water
  • Flush again to rinse it

To maintain cleanliness, throw in some toilet cleaning tablets. These sit in the cistern and slowly dissolve into the water creating a disinfecting solution that keeps the cistern and the toilet clean for longer.

Clean Your Toilet More Often

Cleaning your toilet more often makes it harder for any dirt, mold, or bacteria to take hold. If you are using the correct toilet cleaner, the mere act of adding it to the toilet and scrubbing makes conditions very unfavorable for dirt and gunk.

Using a toilet cleaner once a day with a brush to scrub your toilet should be enough. Every other day should be the bare minimum.

Alternatively, you can buy yourself two days off by using toilet cleaning tablets that condition your flushing water, making it sanitize your toilet every time you flush.

When cleaning, ensure that you apply the cleaner to the bottom of the toilet rim and scrub there. Most infections happen because people forget to clean up there.

Warning: Never mix different toilet cleaners. Mixing could lead to a chemical reaction that releases dangerous gases like chlorine and Florine. Rinse your toilet thoroughly after using a cleaning agent to ensure that there won’t be a considerable reaction when you move on to another cleaner.

Is Black Mold Under Toilet Rim Dangerous?

Black mold is as dangerous as any other form of mold. Inhaling spores from this mold can lead to respiratory system irritation and complications.

The effects are more profound if you inhale many spores or have allergies and asthma. Expect worse complications if you have other underlying respiratory health complications.

What is the Black Stuff in the Toilet After Flushing

While the mold under the toilet rim could stain the toilet water after flushing, black stuff every time you flush could mean contamination from within the cistern.

It could be anything from a deteriorating black rubber fitting to something a person dropped into the cistern intentionally or accidentally.

Open the cistern and inspect it for any residue or deterioration that needs fixing before things escalate and you end up with a leaky cistern.

Bottom Line

Keeping your toilet clean with regular cleans using an appropriate toilet cleaner, and a brush should be good enough to keep those dark stains away.

However, you should schedule some deep cleaning sessions now and then. Spend extra time cleaning and decalcifying problem places like under the toilet bowl or getting hard water deposits using a pumice stone to give your toilet the perfect shine.