Black Mold in Toilet Bowl and Tank: Causes and How to Remove It

It is not uncommon to see black mold in a toilet bowl or tank. It is unsightly and can be a hazard to your health. You need to remove it immediately!

black mold in toilet bowl and tank

Luckily, treating it is not a tough thing to do, and there are also some ways to prevent it from growing again. Let’s dive into this topic, explore the causes of this problem and learn how to remove this annoying black mold in your toilet.

Causes of black mold in toilet tank and bowl

Mold is a type of fungus, and some molds have a black color. But in many cases, people refer it to a specific mold called Stachybotrys chartarum (S. chartarum).

Black mold likes to grow in wet, dark, and warm places, and the toilet is just a perfect breeding ground.

causes of black mold in toilet bowl

There are a few causes of black mold in your toilet:

1. You leave the toilet lid closed when not in use. Black mold tends to thrive in a dark place. If you forget to open the lid after each use, the fungus will grow in the bowl.

2. The toilet hasn’t been used for a long time. You often see a black ring in the toilet bowl or black mold in the tank after a vacation. The stagnant water provides a good place for black mold to grow.

3. The water in the plumbing system has black mold. When you live in an old house, it is possible that the household plumbing system is rusted and has mold growing in the pipe. The water quality in some areas is also questionable. 

4. The waste contains sugar. Black mold grows fast when it comes across sugar. If the waste of users hasn’t been flushed completely, the black ring will form in the bowl quickly.

5. The washer in the tank is broken. The washer works to prevent water from going through. If you find black mold on the bottom of the toilet tank, there may be a leak that you haven’t noticed yet.

Black mold can lead to coughing, shortness of breath, chest tightness, even other serious symptoms, making it a thing you need to remove as soon as possible.

How to remove black mold in a toilet tank and bowl

remove black mold in toilet

Before you get your hands on the removing task, you need to equip yourself with some gears. To prevent mold spores from getting into your eyes, mouth, and nose, you should wear an N95 mask, long gloves, and goggles. 

You should also prepare a toilet brush, a handle brush, and a spray bottle.

No matter which method you use, there are 3 steps you need to follow in advance.

1. Open the door and windows of the bathroom and turn off the ventilation fan if available. 

2. Wear a mask, goggles, and gloves.

3. Turn off the shutoff valve and flush the toilet to remove excess water.

Method 1: Use white vinegar (natural solution)

Sometimes, natural solution works wonder on these chores. Find a bottle of white vinegar in your pantry, and it will help you get rid of the black mold in the toilet.

1. Pour 1 cup of vinegar into the tank and another cup of vinegar into the bowl.

2. Fill the spray bottle with vinegar and spray it to any place that hasn’t been taken care such as the place under the rim of the bowl.

3. Let it sit for 15 to 30 minutes.

4. Use a toilet brush to scrub the surface of the toilet bowl until the black ring and other black stains disappear.

5. Turn on the shutoff valve and flush the toilet a few times to wash off the remaining vinegar and molds.

6. Use a soft hand brush to scrub the wall of the tank to remove any molds inside. Don’t bump any parts inside the tank.

7. Flush the toilet 2 to 3 times. 

8. If you find any black mold left, repeat the process.

Method 2: Use white vinegar and baking soda (natural solution)

White vinegar and baking soda work great on dissolving stuck poops. They also have a good performance in dealing with black mold.

1. Pour 1 cup of vinegar into the toilet bowl and tank.

2. Spray some vinegar inside the bowl and tank, including the place under the rim.

3. Sprinkle baking soda in the bowl and tank.

4. Close the lid and let it sit for 30 to 60 minutes.

5. Use a brush to scrub black mold off. 

6. Turn on the shutoff valve and flush the toilet 3 to 4 times.

7. Repeat it if there are some black molds left.

Method 3: Use bleach

Bleach is a common solution in the household for removing tough stains. It is also helpful in getting rid of molds.

1. If you only need to remove black mold in the toilet tank, you shouldn’t turn off the shutoff valve. Just pour a cup of bleach (1 cup per gallon water) into the tank.

2. Use a handheld brush to scrub the tank. 

3. Flush the toilet a few times to make sure the mold in the tank is gone.

4. Turn off the shutoff valve if the issue is in the bowl and flush to remove excess water.

5. Mix 1 part bleach with 10 parts water and fill the bottle with this solution.

6. Spray the solution around the bowl.

7. Use the toilet brush to scrub the entire bowl. 

8. Turn on the shutoff valve and flush the toilet 2 to 3 times.

Method 4: Use hydrogen peroxide

Even though hydrogen peroxide is for preventing infections in most cases, it could be a good solution to remove black mold.

1. Pour 1 cup of hydrogen peroxide into the toilet tank or toilet bowl.

2. Use a brush to scrub the bowl surface and tank.

3. Let it sit for 15 minutes.

4. Flush the toilet and repeat the process if needed.

Note that you should throw away all the brushes, gloves, goggles, masks in case any mold spores are left on them. You should also take a shower after completing this task.

How to inhibit black mold growing in your toilet

The best way to deal with black mold in the toilet is to prevent it from thriving again. There are 3 simple ways to do it.

Control the humidity in the bathroom

Black mold grows in damp environments. To control it, the most effective way is to control the humidity in the bathroom. 

You can check the humidity level of the bathroom every so often and use a ventilation fan or dehumidifier to reduce the humidity when needed. The ideal humidity should be under 50%. 

Use preventive tablet

You can put a preventive tablet into your toilet tank. The chemicals in the tablet can stop the mold from growing. 

Every time you flush the toilet, the tablet can release some chemicals in the water to keep any molds at bay.

Clean the toilet regularly

Use any solution I mentioned above to clean your toilet regularly. For instance, using vinegar to scrub the toilet bowl and tank twice a week. There may be some mold spores left in the toilet, and you should keep cleaning it.

You should also check any leak and repair it as soon as possible.

Black mold in the toilet bowl and diabetes

As I said, black mold tends to grow fast in an environment with high sugar. Therefore, the toilet bowl becomes a better place for black mold to thrive when the urine contains sugar. 

The urine of people with diabetes has high glucose content, which offers more energy and nutrients for mold spores to breed. 

Therefore, the persistent buildup of black models could be a sign of diabetes. Maybe someone in your family has diabetes.

The black mold could affect your immune system negatively, and you should identify the real cause of it.


Black mold in the toilet bowl and tank is annoying and dangerous. A few days of vacation, an unnoticed leak, or a wet bathroom could lead to the growth of these active fungi.

Luckily, removing it is easy peasy. With the help of vinegar, baking soda, or bleach, these molds will be killed. But to prevent it from appearing again, you should continue to clean it regularly. 

I hope this guide can help you deal with this common bathroom issue.

Avatar photo

Kevin Chow is an enthusiast of home improvement. He is an engineer and likes to test all the tech gadgets on the market. He starts to use bidet seats after his journey to Japan and he had tried a few brands.