Why there's slime in your toilet tank, and how to fix it
Orange, red, or brown slime in the toilet tank is usually iron bacteria. Dark or black slime with a rotten egg smell points to sulfur bacteria. Both are a nuisance, with no drinking water standard. Pink slime on surfaces is an airborne bacteria. Shock chlorination is the usual first fix for well bacteria.
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Step 1: Lift the toilet tank lid and look at the color
Penn State Extension says the back of the toilet tank is an ideal place for this slime to grow. The color tells you which bacteria you're dealing with.
| What you see | Likely cause |
|---|---|
| Orange, red, yellow, or brown slime, or jelly-like strands | Iron bacteria |
| Gray, rusty, or "feathery" growth in standing water | Iron bacteria |
| Dark or black slime, often with a rotten egg smell | Sulfur bacteria |
| Pink film on the toilet bowl waterline, sink, or shower | Airborne bacteria, Serratia marcescens |
Penn State says pink film on surfaces isn't a water problem. See our pink stains page.
Step 2: Notice the smell
Iron bacteria often bring a smell. The Minnesota Department of Health lists swampy, oily or petroleum, cucumber, sewage, rotten vegetation, and musty odors, mostly after water sits unused. Sulfur bacteria make hydrogen sulfide, the rotten egg smell.
You may also see a rainbow, oily-looking sheen on standing water. Minnesota says that can come from iron bacteria too.
Are iron and sulfur bacteria harmful?
They don't cause illness themselves, and there's no drinking water standard for them. Penn State calls them more of a nuisance than a health hazard.
Two cautions from Minnesota: iron bacteria can create conditions where disease-causing organisms grow, and they can clog pumps and pipes and cut your water flow. Minnesota recommends testing for coliform bacteria and nitrate when you find them.
The EPA notes that your local health department may test well water for free.
Step 3: Shock chlorinate the well
Shock chlorination is the most common first step. It puts a strong dose of chlorine down the well and through the plumbing. Minnesota says the concentration should be close to, but not more than, 200 ppm (parts per million), because a stronger mix disinfects less well. Use our shock calculator to work out how much bleach your well needs, and follow our shock chlorination guide.
The bacteria often come back. Penn State says the slime layers protect the bacteria from chlorine, so it can be impossible to get rid of them for good.
If the slime keeps coming back
| Option | What it involves | What to know |
|---|---|---|
| Use more water | Flush the system regularly | The Wisconsin DNR lists this first. |
| Periodic shock chlorination | Shock again when slime returns | Wisconsin lists this as a fix. Penn State says frequent shocking can damage the water system, so if you need it more often than every 6 months, look at other options. |
| Physical well cleaning | Pull the pump, scrub the casing, then treat | Minnesota says this is for heavily infected wells. Done by a licensed contractor. |
| Continuous chlorination | Chlorine pellets dropped into the well on a schedule, plus a whole-house filter | Penn State lists it for persistent cases. Minnesota doesn't usually recommend it, since it can hide other contamination and cause corrosion. |
Minnesota warns never to mix acid cleaners and chlorine. Acid well treatments are for professionals only.
When to call a pro
Call a licensed well driller or pump installer if shocking doesn't help, if your water flow has dropped, or if the well needs to be cleaned. Wisconsin says they have the equipment for well cleaning, and Wisconsin requires state approval before pellet chlorination goes in. Check your own state's rules.
Questions people ask
Is the slime in my toilet tank harmful?
Iron and sulfur bacteria don't cause illness, and Penn State calls them a nuisance. Minnesota still recommends a coliform bacteria and nitrate test, since iron bacteria can create conditions where other organisms grow.
Why does the slime come back after I shock the well?
The bacteria live in slime layers in the well and plumbing, and Penn State says those layers protect them from chlorine. Wisconsin lists periodic shock chlorination as a fix.
How do I get rid of slime in my toilet tank?
Scrubbing the tank removes what you can see. If the bacteria live in the well, the slime returns. Penn State says shock chlorination of the well is the usual first step, and for heavily infected wells Minnesota starts with a licensed contractor cleaning the well and pump.
Where this comes from
- Penn State Extension, Iron and Sulfur Bacteria, a Slimy Problem
- Minnesota Department of Health, Iron Bacteria in Well Water
- Wisconsin DNR, My Water Leaves Slime in Toilet Tank
- Penn State Extension, Common Drinking Water Problems and Solutions
- Minnesota Department of Health, Hydrogen Sulfide and Sulfur Bacteria in Well Water
- EPA, Protect Your Home's Water