Black Mold in Toilet Bowl & Tank

Black mold is common and can grow and various different surfaces. When it begins growing on a toilet it can be a mystery why it’s growing, and how to get rid of it. It can also return very soon after you remove it, so today I will explain how to get rid of black mold in your toilet for good.

As a general rule, use bleach to kill the black mold. If the mold is particularly hard to remove with a brush, it’s perfectly safe to use sandpaper on a toilet bowl. Sandpaper is much more effective at removing it. To stop black mold from returning, pour bleach into the water tank.

Bleach can damage the components in the water tank, therefore, it’s best to empty the water tank and keep it empty by holding up the float so the tank doesn’t refill. Below, I will explain in detail how to run bleach through your toilet to kill mold completely.

Why Black Mold Grows in Toilets

According to the CDC, black mold grows in conditions that are damp, but also within a certain temperature range, between 40°F to 100°F (4.5°C to 38°C). This is quite a wide temperature range and is common in virtually all regions. It also requires a humidity of 70%. This is easily achieved inside the rim of a toilet.

close up view of toilet jet holes
Image from SMART Plumbing Hawaii

As water enters the bowl, it goes through the top of the bowl, and then out through the openings underneath the lip of the toilet bowl. Mold will typically take root inside the toilet above the openings. 

Therefore, after removing the mold inside the bowl, it will grow back and be visible around the rim again. For this reason, you need to kill the mold that is growing on the interior of the bowl. That can’t be reached with a toilet brush. To do that it’s best to run bleach through the toilet.

To shortcut this process you can simply scrub off any visible black mold. But, to permanently remove use the following method.

Using an inexpensive water pump to kill all mold

This method requires an inexpensive water pump, duct tape, bleach, and a hose. Here’s a really good video that shows how to do it:

Mold is a microscopic plant, that grows roots into what it’s growing on. If you apply bleach to just the surface of the mold one time, it can kill the top of the mold, which will die and fall off. But, unless you soak the mold in bleach using the pump method shown above, it’s very likely it will survive and regrow.

Mold grows via spores. These are microscopic seeds that can be present in the air, and water. Once, some spores successfully latch onto a surface and take root they will begin to grow. Regularly running bleach through your toilet can be very effective and killing mold while it’s still young and weak. For example, you can flush some bleach through the toilet once every few weeks.

Mold can also grow on the underside of the toilet seat. Many toilet seats are made of a plastic material that is easier for mold to take root on.

Removing mold once it has been killed with bleach

Black mold is particularly resistant to brushing. A regular toilet brush or similar will require A LOT of scrubbing to remove the mold. 

A better option is to use something more heavy-duty. It’s a little-known fact that sandpaper can be used on a toilet bowl and won’t damage it at all. Sandpaper is far more effective at removing mold than a brush. It’s best to get use a wet and dry sandpaper.

Because waiting for your toilet bowl to get completely dry takes quite a bit of time. And, keeping the mold wet will soften it, and make it easier to remove. But, don’t use sandpaper on the underside of the toilet seat. It will scratch it. Use a hard-bristled plastic brush. It’s also a good idea to soak mold on the underside of the toilet in bleach. 


This can be done by soaking some toilet paper or paper towels in mold and sticking them to the toilet seat using some duct tape. Then let it sit, and keep applying bleach to it once it dries out. Generally, 2 to 6 hours should be enough for it to completely soak into the mold and kill it.

Is Black Mold in a Toilet Bad

A perfectly clean toilet with no black mold or discoloration looks the best. However, black mold is particularly difficult to remove, and therefore you need to allow some time to remove it. Because of this fact, black mold can be left on your toilet for a few days or a week, but is black mold harmful?

Overall black mold in a toilet is not bad. However, people can have allergies to black mold. Prolonged exposure to black mold can produce flu-like symptoms. But, a bathroom is only used for short amounts of time.

As a general rule, it’s good to keep your bathroom or a separate toilet well ventilated. This applies double if you currently have black mold in your toilet. As black mold grows it releases spores that can cause hayfever, and flu-like symptoms when inhaled.

Why Black Mold Keeps Growing on a Toilet

After you remove black mold, it can continue to grow back again. Because it’s very difficult to remove from a toilet it’s important to know why it keeps growing back. Here’s why black mold keeps growing back specifically on a toilet. 

pouring bleach down the toilet

In general, black mold will grow whenever the humidity is above 70%, and the temperature is between 40°F to 100°F (4.5°C to 38°C). Black mold is easily killed with bleach. So, apply bleach to your toilet regularly to kill black mold before it begins to grow.

Doing so will kill it before it grows enough that it becomes visible and needs to be removed. Generally, pour bleach through your toilet by adding it to the water tank about once a week. It can also be a good idea to get a digital thermostat that displays a humidity reading. 

That way you can see when the humidity is around 70%. If it isn’t then you don’t need to be as concerned with black mold growing and can take a break from applying bleach to your toilet.

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