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What Causes the Blue Stain in the Toilet Bowl, Plus Solutions

Have you caught yourself staring down at your toilet, asking, “What causes the blue stain in the toilet bowl?” It was my first thought when I saw it in my own bathroom! Identifying that blue stain is the first step to getting rid of it.

It can take some trial and error to narrow down the cause of the blue stain in the toilet bowl. With the proper knowledge, you should be able to remove it with ingredients you already have at home.

What Causes the Blue Stain in the Toilet Bowl

What Causes Blue Stain in Toilet Bowl?

The most likely cause of a blue stain in the toilet bowl is because of blue dye. The popular blue disinfecting tablets you put in your toilet tank dye the water blue. When the blue water isn’t flushed enough, it can stain the porcelain.  

Other Possible Causes

Cleaning tablets may be the most likely and easy to clean cause, but what if you’ve never used those tablets?

In my experience, other causes can be a bit more challenging to check for. They can range from hard and acidic water with unfiltered metals and minerals or mold in the water.

  • Breakdown of copper pipes over time can lead to blue stains in the toilet bowl. Acidic water in copper pipes breaks down your lines resulting in a blue pigment.
  • Surface cleaners that use a strong blue dye can leave a residual stain in toilets.
  • Hard water is also a likely culprit. Most of our water supply flows through rocks and soil; it can pick up minerals such as calcium.
  • Penicillium mold.

Water Filtration Issues

Water filtration issues include excess minerals in the water and pipe problems.

A sound water filtration system will remove many impurities that could cause problems.

There are many options for water filtration systems. You’ll find single units to complete house systems. Yet, these are costly, so cleaning the blue stains each time may be cost-effective in the long run.

Unfortunately, no system can remove all impurities from water. Sometimes you need to treat the water before it’s filtered. You can choose to clean the blue stains as needed. Or you can get water filters as an option.

Acidic Water

Acidic water is corrosive to pipes, with a pH of 6.5 or less. It contains heavy metals such as copper, zinc, manganese, and iron. You can determine if your water is acidic by contacting a state-certified laboratory.

The Northeastern United States is most likely to have acidic water. This is due to its concentration of population and industrial complexes. Additionally, soil, rock formations, and tree roots may make the groundwater acidic.

Along with stains, acidic water causes health problems. Drinking or coming into contact with heavy metals over a long period can cause:

  • Vomiting
  • Diarrhea
  • Kidney disease
  • Liver disease

Filtering the water will prevent further staining. It will also remove heavy metals from your drinking water.

Water Filtering

Blue stains in the toilet bowl are often the first sign that the water is becoming acidic. So, as well as removing them, treat them like a warning sign to prevent getting sick. When acidic water damages copper pipes, they also become weaker over time. This corrosion will cause fractures, leaks, and permeability.

Then you can remove the remaining stains with ingredients already found in the home. 

  • Apply a paste of baking soda and white vinegar to the stain. Rinse the paste off and repeat, if necessary.
  • Pour rubbing alcohol on a paper towel and rub the stain. Repeat the process if it does not remove all of it at once.
  • Chlorine bleach can also remove the stain. Use equal parts water and bleach on the blue stain to remove it and repeat if necessary.


The use of both galvanized and copper metals in plumbing lines causes electrolysis. This is a fancy term for chemical decomposition caused by electric currents and ions passing through a liquid.

When the pipes meet, the lower noble copper metal will rust. This rusting causes degradation and weakening in the metal. As a result, the blue stains will appear outside the joints where the lines meet.

I always say to get a professional when it comes to plumbing. Contact a plumber for recommendations on changing your pipes to prevent future stains. To remove stains, apply a paste of baking soda and vinegar or a diluted bleach solution. Always use the manufacturer’s instructions in the toilet bowl.

Hard Water

Hard water is water with high mineral content. It’s most common in the Southern and Midwestern parts of the United States. The minerals come from gypsum, limestone, chalk stone, and calcium and magnesium concentrations.

Minerals can cause household issues, like yellowed clothes, plugged appliances, and clogged pipes.

If you notice a film after washing your hands, residue on your sink, or buildup inside your laundry machine, your water may be hard.

Hard water is usually more annoying than a threat to your health. Filtering the water before entering your house with a primary line filter. This can help to prevent mineral accumulation and future problems.

Removing blue stains caused by hard water involves an extra step. Remove any mineral crust left on your toilet before treating it with baking soda and vinegar.

A few ways to remove the crust without scratching the porcelain toilet bowl are:

  • Use a sturdy plastic scraper
  • Vinegar to dissolve the crust
  • Apply a paste made from hydrogen peroxide and baking soda to break it apart
  • Fluoride toothpaste applied with a toothbrush will remove the crust.

Penicillium Mold

Penicillium mold occurs everywhere in the environment. But, it often grows in the warm and humid Southern coastal region of the United States.

Molds enter the home carried in on pets, clothing, and shoes or through doors, windows, and vents. Bathrooms are a target since warm and damp environments are perfect for mold growth.

Preventing mold growth by dehumidifying your home is critical to preventing blue stains. Venting fans from the kitchen, the clothes dryer, and bathrooms outside the home are all great first steps.

You can also try keeping the humidity level in the house to less than 50% by purchasing a dehumidifier. Run the fan or an air conditioner when showering and or a time afterward.

Removing mold from your toilet isn’t an aesthetic issue; it’s also a health one. Clean mold deposits with an Environmental Protection Agency-approved clean such as Moldex.

Remove mold stains with a mixture of 1 cup bleach to 1 gallon of water in a well-ventilated area.

Penicillium Mold

Blue Dye Cleaners

If you use blue tank tablets, rinsing the toilet is essential. Many blue cleaners contain hydrochloric acid or ammonia and can leave behind residue. Wash your toilet with soap and rinse well with water before and after.

To remove any lingering stains:

  • Scrub with either powder or liquid chlorine bleach.
  • Use a Mr. Clean magic eraser.
  • Apply a baking soda and vinegar paste.

Unfortunate Uncleanable Causes

Sometimes, you can’t remove stains. You have to bite the bullet and replace something.

Conditions that can stain your toilet blue are: 

  • Chromhidrosis
  • Pregnancy
  • Raised levels of hormones like progesterone and estrogen

These are medical conditions, not problems with the pipes or water filters. The toilet stains will reoccur unless the hormonal imbalance is resolved.


So, what causes blue stains in your toilet bowl? 

Unfortunately, many things can. Sometimes it’s hard or acidic water. However, for the most part, blue toilet cleaners are a prime suspect. It’s important to investigate the reason why your toilet is blue before you invest in a solution.

Try home solutions like baking soda, vinegar, and Mr. Clean magic erasers. They are usually safe, popular, and effective ingredients. Don’t mix cleaners and wipe your toilet clean with water after any attempts to fix your problem.

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