What is the black “Stuff” in my toilet, shower, or pet’s dish?

Each year, a few Woodinville Water District customers call to ask about a slimy black substance that sometimes forms in moist areas around their homes. They most frequently observe it in toilet bowls, on the surfaces in shower stalls and bathtub enclosures, in sinks, and in pet water dishes.

Black Fungus/Slime

A black fungus or mold is thought to be the cause of the black slime. These types of fungi are common inhabitants of our environment and can be found in many places, including human and animal feces, dust, soil, and in surface water. The mold will grow in any moist location where phosphorous containing materials or fatty substances accumulate. Sources of these substances include soap residues in bathing areas, feces in toilets, soap and food residues in pet water dishes. The fungus can also grow in tap water in locations such as toilets in guest bathrooms where the water is left standing long enough for the chlorine residual disinfectant to dissipate.

Certain types of normal environmental bacteria may also grow with it. The fungus is not known to cause any waterborne diseases.


Once established, the growth usually cannot be eliminated entirely. However, periodic and thorough cleaning of the surfaces where the slime occurs, followed by disinfection with chlorine bleach appear to be the best way to control it. First, scrub the surfaces where the mold grows or where phosphorus and fatty substances accumulate with a brush and a household cleanser. Then disinfect the surfaces where the slime grew with a strong chlorine bleach solution. Leave the disinfectant solution on the affected surface(s) for 10 to 20 minutes before thoroughly rinsing it away with clean water.

To control black mold in toilets, clean the bowl thoroughly and spray chlorine bleach into the bowl and under the bowl rim. Also add ¼ cup of bleach to the toilet tank. Let the bleach stand for 15 to 20 minutes. After 15 to 20 minutes, flush the toilet a couple of times to rinse the disinfectant out of the tank and the bowl. Number e: Bleach should not be left in the toilet tank for prolonged periods; it will damage the rubber valves and seals inside. Whenever the black slime starts to reappear, repeat the cleaning and disinfection process.


If you have any questions contact Tim Cantwell our Water Quality/Cross-Connection Specialist at 425-487-4125 or by email.

Show All Answers

1. My water tastes funny. Why?
2. My water smells strange. Why?
3. Is the water fluoridated?
4. Is my water hard or soft?
5. Why is my water discolored?
6. What is the black “Stuff” in my toilet, shower, or pet’s dish?
7. What is the pink “Stuff” in my toilet, shower, or pet’s dish?
8. What is the white stuff in my water?