Pink slime: the bacteria people mistake for algae

What is the pink stuff, if it is not algae?

It is a bacterial biofilm. The organism most often blamed is a Methylobacterium, and it protects itself the way most biofilms do, by generating a slimy matrix that keeps sanitizer from reaching the cells inside. Feel it and it is slick and greasy rather than dusty.

That matters because it changes the treatment completely. Algaecide is formulated for algae. It has essentially no useful effect on a bacterial slime, so the owner who reaches for the algaecide bottle gets nothing and concludes the pool is haunted.

It very often travels with white water mold, which looks like shredded tissue paper or bits of a wet napkin floating near the returns or clinging inside the skimmer. If you see both, you have one problem with two appearances, and the same treatment addresses both.

Where does it hide?

Anywhere water is nearly still and sanitizer barely reaches. The inside of the skimmer throat and behind the weir door. Under the lip of the light niche. Inside ladder sockets and the hollow legs of a rail. The underside of steps. Behind and inside a chlorinator or a length of flex pipe that never sees full flow.

It also lives in plumbing, which is why it can appear to come back from nothing after a good scrub. A section of line that is valved off, a spa loop that only runs when someone uses it, an unused water feature line, all of those are reservoirs.

Above the water line matters too. It will grow on the wet band under the coping and around the tile in a spa. If a spa has sat covered and warm at a weak sanitizer level, that is the classic setup for it.

How do you actually clear it?

Physically first. Scrub every affected surface with a brush, get into the skimmer, pull the weir door, take out the ladder if it comes out, and open up the light niche if you can do that safely with the power off. Biofilm has to be broken up mechanically before any chemical will reach the organism.

Then raise free chlorine substantially and hold it, not for an evening but for a day or more, with the pump running continuously so the elevated water actually gets pushed through every line, including the spa loop and any feature line you normally leave closed. Open those valves during the treatment. That is the step that reaches the plumbing.

Then deal with the filter. A cartridge should be pulled and cleaned or soaked, a DE grid set cleaned and recharged, a sand filter backwashed thoroughly. Biofilm in the filter is a fresh inoculation waiting for the chlorine level to come back down.

Then fix why it happened. Pink slime is a low sanitizer and low flow disease. If it grew, something in the pool was running at effectively zero chlorine for a while, or a section of plumbing never moved.

Why does it keep coming back in the same place?

Because the reservoir was never reached. If the biofilm is inside a length of pipe or in a valved off loop, treating the open water clears what you can see and leaves the source intact. It reappears at the fitting nearest the reservoir, which is why owners describe it as coming back at the same return or the same skimmer.

Because the filter was not cleaned. A cartridge with slime in the pleats reseeds the pool every time the pump runs.

Or because circulation genuinely does not reach that corner. A return eye aimed the wrong way, a step area with no flow, or a pump running at a speed too low to move water anywhere but the shortest path. Aim the eyes, run the pump longer, and brush the dead corner weekly.

Is it pink slime or is it a stain?

Run a thumb or a white cloth across it. A biofilm smears, lifts and leaves clean surface behind. A stain does not move at all no matter how hard you push, because it is in the surface rather than on it. That one test separates most of these questions in about five seconds and it costs nothing to try before you buy anything.

Texture and location settle the rest. Slime is slick and greasy and it lives where water sits still, so it turns up in the skimmer throat, around fittings, under a step lip and in a spa. A metal stain is flat, it follows the shape of whatever caused it, and it usually sits on open plaster where nothing is sheltering it. A thin iron film on white plaster can look pink in bright sun, which is where most of the confusion starts.

There is also a harmless version. Fine pink grit in a spa filter that rinses out and never returns was dust or a bit of oxidized metal, not an organism. Anyone searching what is the pink slime in my pool and spa should watch the spot for a week or two after cleaning it. This regrows. That is the whole tell, because what you scrubbed away was only the visible part of something living deeper in the system.

Treat it as a report on the rest of the pool

The organism needs almost no sanitizer to get going, so a visible colony means chlorine went to nothing somewhere for a stretch. Before you congratulate yourself on the scrub, find the stretch. A tripped breaker after a storm, a timer that lost its schedule in an outage, two weeks away, a feeder that ran dry, or a cell producing far less than the panel claims.

Verify the sanitizer rather than trusting the display. On a salt pool, test the water rather than reading the controller, because a tired cell will report a normal output and put out very little, and the pool that quietly ran at half strength for a month is exactly the pool that grows this.

Check the schedule too. A short winter run time, or a variable speed pump left on the lowest setting for economy, can move water without ever pushing much of it through the far end of the system. Circulation on paper is not circulation in the skimmer throat, and this is the organism that finds the difference.

Anyone who has had to look up what is the pink slime in my pool and spa has already paid for the expensive half of the lesson. The cheap half is fifteen minutes with a test kit and a look at the timer, confirming that sanitizer is being made and that it is reaching every loop in the plumbing.