The Causes And Solutions To Black Mold In Toilet Bowl And Tank

Black mold, which despite its name can often appear orange or green, is a type of fungus that grows in moist, warm and dark locations. This is why black mold is quite common to see in bathrooms due to the humidity, especially when ventilation is poor.

What Is Black Mold?

I’ll start this guide by explaining the causes of black mold in your toilet. It’s critical to understand why this might be happening first, as prevention is as important as the solution.

Causes Of Black Mold

It’s important that I first clear something up. Brown stains which are commonly found in toilet bowls may be a separate problem, which is often due to poor water quality.

Causes Of Black Mold

When water remains stagnant in locations like a toilet bowl, mold can rapidly grow. As a toilet bowl by its very nature has stagnant water, this problem can quickly occur. This is because of the warm and moist environment which increases the risk of chlorination.

Infrequent Flushing

Mold is a type of fungus that does not require chlorophyll to grow (which would normally come from the sun). As such, when the fungus is left in darkness (like a closed lid), it finds a perfect opportunity to thrive. 

Dark Areas

Mineral deposits can actually feed mold as it acts as nutrition. Mineral deposits are abundant in the walls of the tank and in stagnant, hard water. Of course, the more minerals that are present – the more nutrients the mold will receive and the more it will grow.


When a toilet still has human waste present (whether you can see it or not), mold is encouraged to grow. This is especially the case if the human waste contains a lot of sugar, as mold grows quicker when exposed to sugar.

Residual Human Waste

The Causes And Solutions To Black Mold In Toilet Bowl And Tank