It’s an inevitable issue that nearly every comes across at one point or another.

You’re about to use the bathroom, when you notice something disgusting. There’s a weird black ring inside of your toilet, and it’s looking pretty gross and gnarly. Generally, it’s an easy enough fix with some toilet cleaner and a brush. But all the same, it keeps coming back.

So what is it? Why does it keep showing up? Is there anything you can do to prevent it?
Here’s a few reasons why it happens, and what you can do!

What Are the Causes?

Mold & Mildew

Mold and mildew are both a type of fungus that thrive in dark, damp areas, like the underside of the edge of your toilet bowl where the water flows. These are types of spores that can be potentially dangerous if inhaled. The rings can appear black in color, though they can also appear brown, orange or green as well. Mold should be removed immediately upon discovery to prevent it from spreading.


A type of bacteria that grows in damp and dark areas, like toilet bowls is serratia marcescens. It can leave a pink colored ring in your toilet, as well as your sinks and tubs if left unchecked. The bacteria are considered harmful to humans, and have been known to cause wound infections, urinary tract infections, and pneumonia. The bacteria also have antibiotic resistant properties that can make treatment of an infection difficult. Bacteria should be removed immediately upon discovery to prevent any potential harm.

Mineral Deposits

Hard water contains tiny minerals that can collect in areas with a heavy flow. These minerals appear as black, gray, or brown lines and rings in your toilet bowl, creating water lines where the flow is strongest. If you have issues with hard water in your home, this is probably one of the culprits. While not an immediate health hazard, it’s best to remove mineral deposits sooner than later to prevent a larger buildup from occurring.

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What Can You Do?

Usually, a standard cleaning of your toilet will remove most buildup. This only requires a toilet brush and a toilet cleaning solution, preferably one with bleach to kill bacteria.

If you’re serious about making sure you prevent those black streaks from appearing, you may want to consider this more in-depth cleaning method.

  • Gather together some bleach, a measuring cup, and your toilet brush
  • Flush your toilet to bring in a fresh flow of water
  • Pour 1 cup of bleach into the bowl
  • Stir the water in the bowl with the brush and thoroughly scrub the streaks away
  • Wait 10 minutes before flushing to allow the bleach to kill all mold and bacteria
  • Flush two times in succession to remove all bleach and remainder

This method, while more time consuming, is a guaranteed way to prevent black streaks from building up in your toilet for a while.

Many stores sell bleach tablets that you can drop into the tank of the toilet, which keeps your toilet clean for around a month at at time. You can also purchase a clip-on bowl freshener that dispenses cleaning solution with every flush, and will last roughly a couple weeks to a month.

If you need more help, consider calling our drain cleaning team. Our team specializes in fixing the problems that stump other plumbers, and we’re always happy to help with a new plumbing puzzle.