What is white water mold in a pool?
White water mold is a whitish, naturally occurring biofilm, sometimes called a water mold, that looks like shredded tissue paper, wet toilet paper, or a mucus-like slime floating in the water or clinging to surfaces. It is not algae, and it is not the toxic black mold people worry about in houses. It is a saprophytic growth, meaning it feeds on dead organic matter rather than on light the way algae does, which is why it can thrive in shaded plumbing and skimmers where algae would not.
It shows up first in the low-flow, low-sanitizer spots: behind the ladder, in the skimmer basket and throat, around return jets, in corners and steps, and inside hoses and the filter. Because it lives on surfaces and inside equipment rather than suspended in open water, a pool can look mostly clear while white water mold quietly colonizes the plumbing. If you scoop some out, it holds together in slimy strings rather than clouding the water like an algae bloom does.
White water mold is strongly associated with biguanide pools (Baquacil and SoftSwim) because those systems carry no chlorine residual to suppress it, but it appears in chlorine pools too whenever free chlorine runs low, circulation is poor, or the filter is dirty. It is a signal that sanitizer and flow are not reaching part of your pool, which is exactly why the fix has to be physical as well as chemical.
What is pink slime, and is it algae?
Pink slime is a bacterial slime, not an algae, even though it is commonly sold and searched as pink algae. It usually comes from a gram-negative bacterium, most often identified as Methylobacterium, and it is closely related to the Serratia marcescens that leaves the pink ring in showers and toilet bowls. It appears as a pink, orange, or reddish film that feels slimy and coats plastic surfaces, ladder steps, light niches, PVC fittings, and the waterline.
Like white water mold, pink slime concentrates in the same dead spots where water barely moves and sanitizer is weakest, and the two very often show up together on the same pool. Because it is a bacterium wrapped in a slime layer rather than a plant, it does not float as a suspended bloom. It builds as a coating you have to wipe or brush to feel, and it returns fast if you kill the surface film but leave the colony in the filter or a fitting.
Pink slime is not something to swim in. It signals that the water is under-sanitized, and while it is not as dangerous as it looks, Serratia and similar bacteria can cause irritation or infection through cuts, eyes, or ears in susceptible people. Treat a visible pink film as a stop-swimming sign until the water is cleared and holding chlorine.
Why won't chlorine kill white water mold and pink slime on its own?
Chlorine struggles with both because they live inside a biofilm, a self-made protective slime that shields the organisms from sanitizer, the same trick that makes black algae so stubborn. Chlorine can only kill what it actually touches, and the outer slime layer takes the hit while the growth underneath survives. This is why pouring in a normal dose does little: the chlorine is consumed at the surface of the film and never reaches the colony.
There is usually a chemistry problem stacked on top of the biofilm. If free chlorine is chronically low, or if your pH is high enough to switch most of the chlorine off, or your cyanuric acid is so high the chlorine is throttled, the sanitizer never had the strength to keep these growths out in the first place. Get pH into the 7.2 to 7.6 range so the chlorine is in its active form before you judge whether it is working; the method is in how to lower pH in a pool, and the target for every reading is in ideal pool chemistry levels.
In a biguanide pool there is no chlorine residual at all, which is why white water mold and pink slime are so common there and why they keep coming back until you convert. If you are fighting these growths on Baquacil or SoftSwim over and over, the real fix is often the switch to chlorine, covered in Baquacil versus a chlorine pool.
How do I get rid of white water mold and pink slime?
Clearing them takes four steps in order: balance the water, physically break up the slime everywhere, shock high and hold it, and clean the filter that is hiding the colony. Start by getting pH down into the low 7s so your chlorine is active, then confirm cyanuric acid is not sky-high, because a high stabilizer level raises the chlorine you need. Do not skip the balancing; shocking into high pH wastes most of the chlorine.
Next, brush and scrub every surface hard: walls, floor, steps, behind and under the ladder, the skimmer throat and basket, return fittings, and the waterline. The brushing is not optional, because it is what tears open the biofilm so chlorine can finally reach the organisms underneath. Pull out and clean or discard any toys, hoses, cleaner parts, and the skimmer and pump baskets, since the slime rides along on all of them and will reseed the pool.
Then shock to a high free chlorine level and hold it there while you keep brushing daily until the slime is gone and the chlorine stops dropping overnight. Because this is a protected biofilm, it needs a strong, sustained level rather than one big pour, so size the target with the shock calculator for your pool and stabilizer, add it in stages with the pump running, and retest before adding more. Return to your normal maintenance level, set by the chlorine calculator, only after the water is clear and holding. The general shock routine is in how to shock a pool.
Do I need to clean the filter and equipment too?
Yes, and this is the step people skip that lets it come back within days. The filter is the number one place both growths hide, because slime-laden water passes through it constantly and the media is dark, wet, and low-flow, which is ideal for a biofilm. Shocking the pool while leaving a colonized filter in place just recirculates the problem, so clean or replace the filter as part of the treatment, not after.
How you clean depends on the filter. A cartridge should come out and get a deep chemical clean, or be replaced if it is old and the slime is heavy. A DE or sand filter should be backwashed thoroughly, and with a bad case it is worth replacing DE grids or old sand and doing a chemical rinse, since backwashing alone may not clear an established film; see how to clean a pool filter and how to backwash a sand filter. Run the pump continuously through the whole treatment so there are no dead spots, and clean the filter again afterward once it loads up with the dead slime you have just killed.
Do not forget the small stuff. The skimmer weir and basket, ladder bolts and treads, light niche, return eyeballs, vacuum hose, and automatic cleaner all harbor the biofilm, so wipe or soak them in chlorinated water. If even one colonized fitting or a toy left in the water survives, it can reseed the pool and you will be doing this again.
How do I keep white water mold and pink slime from coming back?
Prevention comes down to the two things that let them start: weak sanitizer and dead water. Hold a steady free chlorine level for your stabilizer instead of letting it swing down to zero between doses, and keep circulation strong so no corner of the pool sits still. Aim return jets to move the whole pool, run the pump long enough each day for a full turnover, and point flow toward the shaded low-spots where slime begins.
Keep up the physical maintenance that never let a biofilm establish: brush the walls and dead spots weekly, empty and rinse the skimmer and pump baskets, and clean the filter on schedule rather than waiting for a problem. A pool kept on a consistent routine almost never grows this stuff, so the pool maintenance schedule is the real long-term defense here.
If you keep getting white water mold or pink slime no matter what you do, look at the system rather than the symptom. A chronically dirty or undersized filter, a pump that does not turn the water over, or a biguanide sanitizer with no residual will all keep feeding the cycle. Fixing the underlying flow and sanitizer problem, and converting a biguanide pool to chlorine, is what finally ends it for good.
Frequently asked questions
Is white water mold in a pool harmful?
White water mold is not the toxic black mold people fear in houses, and it is not as dangerous as it looks, but it is not harmless either. It is a nuisance biofilm that signals your water is under-sanitized, and where it grows other bacteria can grow too. Do not swim until it is cleared and the pool is holding a normal chlorine level. It will not clear itself, so treat it rather than wait it out.
Does white water mold go away on its own?
No. White water mold protects itself inside a biofilm and feeds on dead organic matter, so it will keep spreading through the plumbing and skimmer until you physically break it up and shock the pool. Waiting only lets it colonize more of your equipment, especially the filter. You have to brush and scrub every surface, clean the filter, and hold a high free chlorine level to actually get rid of it.
Does pool shock get rid of white water mold and pink slime?
Shock is necessary but not enough by itself. Because both growths live inside a protective slime, a single dose gets consumed at the surface of the biofilm and never reaches the organisms underneath. You have to brush and scrub every surface first to tear the film open, clean or replace the filter, then raise free chlorine to a high shock level and hold it there while you keep brushing, until the chlorine stops dropping overnight.
Is it safe to swim in a pool with pink slime?
No, do not swim in it. Pink slime is a bacterial growth, not an algae, and a visible pink film means the water is under-sanitized. The bacteria behind it, usually Methylobacterium or the Serratia marcescens that causes pink film in showers, can cause irritation or infection through cuts, eyes, and ears in susceptible people. Clear the slime, get the pool holding chlorine, and then swim.
How do I permanently get rid of pink slime (Serratia marcescens)?
Permanent means removing the biofilm from everywhere it hides, not just the visible film. Balance the water, then brush and scrub all surfaces plus the ladder, skimmer, fittings, and any toys or hoses, clean or replace the filter, and shock to a high chlorine level and hold it. After that, keep it gone by holding steady free chlorine, running the pump enough to turn the water over, and keeping strong circulation into the dead corners. On a biguanide pool with no chlorine residual, converting to chlorine is usually what finally ends the cycle.