What Causes the Pink Stains in the Bathroom?

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If you’ve noticed a pink or reddish film around your toilet waterline, on shower walls, or near sink drains, the culprit is usually Serratia marcescens bacteria. A common environmental bacterium that loves damp, soapy places.

What is Serratia marcescens?

Serratia marcescens bacteria are found in soil, water, and dust. In homes, it produces a reddish-pink pigment called prodigiosin. When moisture, warmth, and soap residue are present, this pigment shows up as a nasty pink film or slimy layer in toilets, showers, and bathroom sinks.

Why Does It Grow in Bathrooms?

  • Moisture: Showers, sinks, and toilets stay damp.
  • Soap scum & organic film: Residues provide easy nutrients.
  • Warmth & humidity: Bathrooms often have ideal temperatures.
  • Stagnation: Standing water and low use let colonies form.

Temporary Fixes: Cleaning & Prevention

  1. Scrub regularly: Use a brush and a cleaner with bleach or hydrogen peroxide.
  2. Disinfect dwell time: Spray, let sit several minutes, then rinse.
  3. Dry surfaces: Wipe shower walls, glass, tile, and sinks after use. Shower curtains are notorious for developing pink film. Close the shower curtain after showering to allow it to dry. 
  4. Air movement: Run the exhaust fan during/after showers; crack a window if possible. 
  5. Flush often: Infrequently used toilets are more prone to a pink ring.

Bathroom Ventilation: A Key Step in Prevention

Dampness drives bacterial growth, so proper ventilation is one of the most effective long-term controls. A correctly sized exhaust fan removes humid air, allowing surfaces to dry faster. Reducing bacteria and biofilm formation.

How to Size a Bathroom Fan (CFM)

Fan capacity is measured in CFM (cubic feet per minute) — how much air the fan moves.

  • Basic rule: At least 1 CFM per square foot of floor area.
    Example: 8 ft × 10 ft = 80 ft² → 80 CFM minimum.
  • Fixture boost: Add +50 CFM for each major fixture beyond the toilet (e.g., shower, tub, jetted tub).
    Example: 8×10 bath (80 CFM) with a shower and a separate tub → 80 + 50 + 50 = 180 CFM.
  • High ceilings (>8 ft): Use room volume:
    Required CFM ≈ (Length × Width × Height) ÷ 60
    Example: 8×10×9 ft = 720 ft³; 720 ÷ 60 ≈ 120 CFM.
  • Ducting matters: Long runs, multiple elbows, or undersized ducts reduce performance — upsize ducting and/or choose a fan with higher static-pressure capability as needed.
  • Run-time: Let the fan run 20–30 minutes after showering.

Health Precautions for Sensitive Groups

For most households, Serratia is mainly a nuisance. People with compromised immune systems or chronic lung/respiratory conditions should take extra care.

  • Why it matters: Aerosols from showers/toilet flushing and contact with contaminated surfaces can be a health concern for vulnerable individuals.
  • Immediate response: If pink stains appear on shower curtains, sinks, or toilets, clean and disinfect promptly. Wear disposable gloves and ventilate while cleaning.
  • Reduce exposure: Use the exhaust fan during/after showers, close the lid before flushing, and replace or disinfect shower curtains regularly.

In higher-risk homes, consider professional water testing and adding a whole-house filtration system, including UV disinfection or other whole-house water treatment, such as a KDF Media/Carbon system, or adding KDF filtration media to your existing water softener as an added safeguard against bacteria in well water.

Viqua Whole-House UV system with prefilter


Using KDF Filtration Media for Whole-House Protection

While cleaning and ventilation curb bathroom growth, water treatment can also help control microbial activity at the source. KDF (Kinetic Degradation Fluxion) filtration media — commonly integrated into whole-house filters and water softeners — uses a redox process that disrupts microorganisms and also reduces chlorine and certain heavy metals. Combined with carbon or other media, such as iron filtration systems or in your water softener, KDF can help limit conditions that favor biofilm formation (including pink staining from Serratia).

For a long-term, whole-home approach, adding a system with KDF filtration media provides an extra layer of protection alongside good cleaning and ventilation practices.

👉 Read my in-depth guide to KDF filtration media.

Long-Term Solutions: Addressing the Root Cause

  • Deep-clean fixtures: Include toilet tanks and under the rim where colonies persist.
  • Add disinfection tablets to the toilet tank. 
  • Ventilation upgrade: Replace undersized fans and correct ducting limitations.
  • Fix leaks: Eliminate chronic moisture from drips and sweating pipes.
  • Whole-house treatment: Consider KDF/carbon filtration, adding KDF filtration media to your current water softener, and adding a post-filtration whole-house UV disinfection system to reduce bacteria and support cleaner plumbing.
  • Routine schedule: Clean/disinfect before pink film is visible; keep fan run-times consistent.

 

The “pink ring/slime” is not rust or mold — it’s Serratia marcescens bacteria thriving in damp, soapy environments. Scrubbing and disinfecting provide quick relief, but pairing that with proper ventilation (correctly sized fan), moisture control, and, when appropriate, whole-house treatment with KDF filtration media and/or a UV filtration system offers the best long-term defense — especially for households with vulnerable individuals.





I wish you Good days and Good water!



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