The pink or reddish slime that shows up on pool walls, ladders, and waterlines isn’t actually algae. It’s a bacterium called Serratia marcescens, and while it’s not dangerous for most healthy swimmers, it can cause infections in certain people, particularly those with weakened immune systems, open wounds, or who wear contact lenses. It also signals that your pool’s sanitation has slipped, which means other harmful organisms could be thriving too.
What “Red Algae” Actually Is
Despite the name pool owners give it, red algae is a pigmented bacterium that produces a pink-to-red color as it grows. Serratia marcescens thrives in moist environments and forms a slimy biofilm that clings stubbornly to surfaces. Run your hand along an affected wall and you’ll feel a slick film that shouldn’t be there. This biofilm is part of what makes it tricky to eliminate: it acts as a protective shield, letting the colony resist chlorine levels that would kill free-floating bacteria.
You’ll typically find it in areas with poor water circulation, like behind ladders, in skimmer baskets, around return jets, and along tile grout. It can also colonize pool toys, PVC tubing, and filter housings.
Health Risks for Swimmers
For a healthy adult with no open cuts, a brief swim in a pool with early-stage pink slime is unlikely to cause a serious problem. But Serratia marcescens is an opportunistic pathogen, meaning it waits for a vulnerability and then takes advantage of it. The infections it can cause range from mild to severe:
- Eye infections: Redness, pain, and a condition called keratitis, especially in contact lens wearers who swim without goggles.
- Urinary tract infections: Pain when urinating, pink-tinged urine, and frequent urges to go.
- Respiratory infections: Cough, shortness of breath, chest pain, and pink-tinged mucus if water is inhaled or aerosolized.
- Wound infections: Any open cut, scrape, or surgical incision exposed to contaminated water can become infected.
- Ear infections: A common entry point for bacteria in recreational swimmers.
In rare and more serious cases, Serratia marcescens can lead to bloodstream infections, bone infections, or inflammation of the heart lining. These outcomes are almost exclusively seen in people with compromised immune systems or in hospital settings, not backyard pools. Still, young children, pregnant women, and anyone on immunosuppressive medication should stay out of a pool showing visible pink slime.
Why It Shows Up in Your Pool
Three conditions fuel a pink slime outbreak. The first is high phosphate levels in the water. Phosphates act as food for bacteria and algae alike, and they enter your pool through lawn fertilizer runoff, decomposing leaves, sweat, sunscreen, and even some municipal water supplies. The second is poor filtration. If your filter is clogged, undersized, or not running long enough each day, contaminants accumulate instead of getting removed. The third is stagnant water. Dead zones with little circulation, like corners and behind ladders, give bacteria a calm surface to colonize.
Low or inconsistent chlorine levels tie all of these together. A pool that dips below 1 ppm of free chlorine, even for a few hours on a hot day, gives Serratia marcescens a window to establish itself. Once its biofilm forms, standard chlorine levels may not penetrate it, which is why prevention matters more than treatment.
How to Get Rid of It
Eliminating pink slime takes more effort than a normal algae cleanup because of that protective biofilm. You need to physically break the biofilm apart before chemicals can do their job.
Start by scrubbing every affected surface with a stiff-bristled brush. Soft brushes won’t cut it. Work in sections, spending at least ten minutes per area, and pay special attention to corners, seams, ladder joints, and anywhere the slime is visible. Missing even a small patch means the colony can regrow within a week.
After brushing, shock the pool at triple the normal dose. For a standard chlorine pool, normal shock is roughly one pound of product per 6,000 gallons, so you’d use three pounds per 6,000 gallons. Turn off the pump and let the pool sit overnight so the high chlorine concentration stays in contact with surfaces rather than dispersing through the circulation system. The next day, run the pump continuously and vacuum the dead material off the bottom.
Plan to brush and vacuum at least every other day for the following week. Clean or replace your filter media, since the biofilm can colonize filter cartridges and sand beds. If you skip this step, you’re reintroducing bacteria every time the pump runs.
Keeping It From Coming Back
Consistent maintenance is the only reliable prevention. The CDC recommends maintaining a minimum free chlorine level of 1 ppm and keeping pH between 7.0 and 7.8. Test both at least twice a day during heavy use. In practice, most pool owners should aim for 2 to 3 ppm of free chlorine as a comfortable buffer against dips caused by sunlight, heat, and bather load.
Brush your pool walls weekly, even when they look clean. Serratia marcescens can establish microscopic colonies that aren’t visible yet, and regular brushing disrupts them before they gain a foothold. Run your pump long enough each day to turn over the full volume of your pool at least once, typically 8 to 12 hours depending on your system.
Reducing phosphate levels cuts off the bacteria’s food supply. Phosphate remover products are widely available at pool supply stores. If you’re getting recurring outbreaks, test your phosphate levels and treat if they’re above 100 parts per billion. Also check your water source: fill water from wells or certain municipal systems can introduce phosphates every time you top off the pool.
Adding an algaecide with a high concentration of quaternary ammonium compounds provides a secondary layer of protection, especially as temperatures climb in summer. These products work preventively, stopping growth before it starts, and are typically added every one to two weeks depending on the product.
When the Pool Is Safe to Swim Again
After a shock treatment, wait until free chlorine drops back to 3 ppm or below before swimming. More importantly, visually inspect every surface. If you can still see pink discoloration or feel any slick film on the walls, the biofilm hasn’t been fully eliminated and you should brush and shock again. A pool that tests at proper chlorine and pH levels, shows no visible slime, and has clean filter media is safe to use.