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

Many toilets may have noticeable dark spots, residue, or a ring inside of the bowl, or perhaps marks behind, in, or under the tank. This is likely mildew or mold. The appearance of these black marks may change to green or orange but will be black most of the time. 

Black mold in the bathroom is caused by a few factors: still water, poor ventilation, and a humid environment. While it can turn your bathroom into an eyesore, it’s not an uncommon problem and one that has a few simple solutions. 

In this article, you’ll get some easy tips to fix black mold, along with some ways to prevent it from reappearing and answers to common questions. 

What is Black Mold Doing in My Toilet? 

Black marks in the toilet – sometimes green or orange – is the result of a fungus that grows in damp, dark areas. As you can imagine, a bathroom is a perfect environment for this fungus to grow. 

Black mold will often appear on bathroom curtains, sinks, tubs, tiles, grout, around the shower, and in toilet bowls and tanks. 


Unlike brown stains that may appear due to quality levels of cistern water, toilet mold is due to moisture. Below are its most common causes: 

  • Toilets that are not used often will often be victims of mildew. They will be warm and moist, which are the ideal conditions for mold to appear. Since these toilets are not often flushed, mold can continue to grow unchecked. 
  • Stagnant water – Inside the tank is not often checked or cleaned. As a result, there is a safe area available for mold and mildew to flourish and feed on the algae provided by stagnant water in your cistern. 
  • Lack of light – Of course, since toilet tanks are usually closed, another environmental factor that mold enjoys will be present. 
  • Mineral deposits – Tank walls and water deposits give mold a further way to thrive, providing nutrients. If you are in an area using a hard water system, these problems will be exacerbated further. 
  • A washer that needs replacing may also be a factor in mold development. 
  • Human waste – This can also cause mold to grow, as it can feed on the sugars provided in human waste
  • The water supply from your piping and sewerage can also be responsible for mold. Rust on pipes can create condensation, which we know is something that mold loves. 
  • Cracks or worn out surfaces in the ceramic of your toilet. Cracks can provide easy locations for mold to grab hold of and develop in your toilet. 

How to Rid Your Toilet Bowl of Black Mold and Spots 

The most usual location for mold in your toilet will be at the bottom or perhaps under the toilet rim. 

If your toilet is not used often, you may notice that mold is developing at the waterline. If this is the case, there is a good chance mold has developed in your tank and this will need to be cleaned. 

If you can see mold around the bowl’s base, there may be a broken water seal. Unfortunately, this will mean a more serious problem that will require the help of a professional plumber since there are problems underneath the toilet, such as leaks, which will provide the conditions for mold to grow. A plumber will have to replace sections of the toilet, such as its wax ring in order to stop leakage. You will then be able to thoroughly clean the bowl and know it will not return. 

Here are the steps to clean mold in your toilet bowl if it’s located around the waterline: 

  • Collect the following items: Buckets, rags that can be thrown away, borax, bleach, baking soda, as well as personal protective wear such as gloves and a mask. Mold is highly toxic! 
  • Open all doors and windows and turn on any fans in your bathroom.
  • Put on your mask and gloves.
  • Flush the toilet and check the locations of all mold and mildew. 
  • Remove and scrub out any water in the toilet in order to get a clear view of the normal water level and toilet base. 
  • Pour in some chemical drain cleaner, starting at the far end of the bowl. 
  • Add borax and/or baking soda. This will help to break down the mold. 
  • Pour or spray vinegar over the baking soda and scrub carefully, using a small soft brush. 
  • Let it set for thirty minutes to an hour. 
  • After this time has elapsed, give the toilet another scrub and then flush. 
  • Pour bleach over the affected areas, then leave again for an hour. 
  • Pour in boiling water and flush. 
  • Hopefully, this will have had a big effect on the mold levels of your toilet. If some still remain, repeat the process again. 

The process above is best if conducted as soon as you notice the first signs of black mold. In order to avoid mold reappearance, clean the bowl and flush regularly. 

Removing Black Mold in the Water Tank 

Be sure to check your water tank occasionally for mold. If you have noticed its appearance, you will need to conduct a thorough cleaning. This can be done in several ways with homemade methods. Below is one such example that may have positive results. 

You will need a soft brush, bleach, and vinegar. 


  • Turn off any water access to the toilet, then flush. This will remove any water in the tank area. 
  • Fill the tank with vinegar to a level where all mold will be affected. Especially check that all corners are coated with vinegar, then leave for half an hour. 
  • After this time has passed, give the mold a gentle scrub, making sure not to damage the tank’s walls. 
  • Turn on the water again and let the water fill up. Add bleach, then leave for a further half-hour. 
  • Flush the toilet again. Hopefully, this will have completely cleaned the mold. If it’s had only a minor impact, repeat the process again. 

Keep in mind that bleach and vinegar are highly acidic products and may affect the tank walls. Be careful when adding these liquids. 

Getting Rid of Mold Behind Toilet Tank 

Water finding its way down the back of the toilet tank can create further issues with mold. Below is a simple homemade solution that will help to fix the issue. 

Before you get started, you will need a bucket, rags that can be thrown out, a soft brush, vinegar, baking soda, and bleach or disinfectant. 


  • Give the back of the tank a gentle cleaning to remove any excess dirt. 
  • Using the baking soda and a small amount of vinegar, make a paste. Remember that the paste will need to be thick enough to stick.
  • Using a paddle-pop stick, place the paste across any mold that has appeared. 
  • Use the hand brush to give the paste a gentle scrub into the mold. 
  • Leave this paste on for thirty minutes. 
  • After this time has passed, scrub gently with the brush to scrape off any mold that has remained. 
  • Use the rag to wipe down the toilet tank. Scrub and wipe down until you can see the process working. 
  • You can then spray the area with tap water. It might also be worth covering the area with bleach.
  • Repeat the process if required. 

I Have Black Stuff Appearing in My Toilet After Flushing – Why? 

If black mold appears after you flush your toilet, there are problems in the pipes that send water to your household. It probably means that there are issues with rust or leaks in the system. These will create black mold that is then being sent into your toilet. 

There is a chance that the blackness may not be mold but an issue with the water. It could be the result of mineral deposits or some other water issue. 

In either of these cases, a plumber will be required to diagnose and solve the problem more accurately. 

Can Toilet Mold Make You Sick? Is it Dangerous? 

Any black mold that appears in your bathroom is a serious health risk to all those in the household. It is hazardous to your health and can result in a range of symptoms such as itchy eyes and skin, wheezing or worse. 

Some individuals have specific allergies to mold spores, and this can, of course, mean their reaction will be even worse. If you have any such allergies, it may be better to have someone else perform the cleaning processes described above or ensure you are using a ventilator in the room and have a mask on. 

Does Black Mold in My Toilet Cause Diabetes? 

Some have asked if the black mold is responsible for diabetes, but no research has shown a direct link. But black mold may be highlighting some diabetes symptoms. 

As stated above, mold loves sugar. Those with diabetes may excrete more sugar when they urinate, creating an environment where mold can flourish. 

For this reason, if you are seeing mold be produced consistently in your toilet, it may mean that you are exhibiting diabetes symptoms. Check with your doctor to discuss getting a test done to check your blood sugar levels. 

Preventing Black Mold in Your Toilet 

There are some simple steps you can follow to make sure that black mold does not make another appearance in your toilet. 

  • Make sure your toilet is properly ventilated, especially during and after a shower. If possible, open doors and windows and keep a fan on to keep the area cool and avoid a lot of steam in the air. 
  • If you are unable to have a window open, consider using a dehumidifier to remove dampness in the room. 
  • Always flush the toilet after every use. 
  • Use scrubs and cleaners to keep toilets clean. 
  • Check occasionally for any leaks or damage to your toilet, and act on these fast before problems are created. 

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