Pink slime

What it is

Pink slime is a naturally occurring bacterium, of the genus Methylobacterium. That single fact explains almost everything about how it behaves and why the usual treatments do so little.

• It is pink-pigmented and forms a heavy, protective slime coating over itself, which gives the organism an unusually high level of protection.
• It is methanol-consuming, so it can feed on sources that have nothing to do with what algae eats.
• It is highly resilient against halogen sanitisers — chlorine and bromine — and against non-halogen germicides as well.
• It frequently occurs together with white water mold, which is a different organism again, covered on its own page.
• It can remain a contaminant even after apparently successful treatment.
• It is not a problem confined to biguanide-sanitised pools, though that association persists. It occurs in chlorine and bromine pools too.

Why it comes back

The reason pink slime returns after treatment is specific and worth understanding, because it changes what you have to do.

This bacterium has an affinity for the surface matrix of PVC plastics. It attaches itself to and into that matrix, which lets it survive out of reach and re-contaminate the water long after the visible slime has gone. That includes an inconvenient list of things: pool toys, floats, ladders and steps, fountains, automatic cleaner parts and hoses, skimmer baskets, weir doors, directional return fittings, garden hoses, and the interior of the plumbing itself.

Very small surviving quantities are enough to re-establish the whole problem. A treatment that cleans the pool and ignores the equipment is a treatment that buys a few weeks.

As a side note that puts its resilience in perspective: in medical laboratory settings this same genus turns up regularly as a contaminant inside tubing — the same behaviour, in a far more controlled environment than a back garden.

What causes it

Pink slime establishes where three conditions coincide:

• Improper or lapsed maintenance — an inconsistent sanitiser residual above all.
• Poor circulation — it prefers slow-moving water, which is why it appears behind fittings and inside skimmers rather than in open water.
• Darkness — it prefers areas not exposed to direct sunlight, which is why the underside of ladder treads and the inside of the skimmer are classic sites.

Prevention

Prevention is very strongly preferred over treatment here, because treatment is laborious and incomplete.

1. Physically brush and clean all pool surfaces weekly, including ladder steps — especially the underside of each step — and rails. Physical disruption of the slime layer is the one thing chemicals cannot do for you.
2. Expose as many surfaces as possible to sunlight. Sunlight and ultraviolet light are genuine natural oxidisers and this organism actively avoids them.
3. Let daylight into the skimmer. Removing the skimmer lid for several hours a day makes the basket a much less hospitable place.

   Safety — inground pools especially. An open skimmer well is a hole in the deck. Do not leave one uncovered where anyone, particularly a child or a pet, could step or fall into it, and never leave one open unattended. If in any doubt, leave the lid on and rely on brushing instead.

4. Keep the water moving — adequate run time and returns aimed to sweep quiet corners. See circulation.
5. Hold a consistent sanitiser residual. Consistency matters far more here than peak level, because this organism exploits the gaps.
6. Do not leave toys, floats and hoses in the water permanently. They are ideal habitat and they shade what is beneath them.

Treatment

Treatment has to be physical and chemical at the same time, and it has to include everything the water touches.

1. Remove everything removable from the pool — ladders, steps, toys, floats, cleaner and hose, skimmer baskets, directional fittings, thermometers.
2. Balance the water first. Sanitiser applied to badly balanced water is wasted, and this organism is resistant enough that you cannot afford to waste any of it.
3. Brush every surface hard, including all the hidden places listed above. The protective coating must be physically broken.
4. Shock aggressively — a full dose for the true pool volume, delivered at once, with the pump running continuously. Partial doses select for the survivors.
5. Soak or wash all removed equipment in sanitised water at the same time, and leave it in the sun to dry afterwards. Anything porous or cracked that cannot be properly cleaned should be replaced.
6. Clean the filter chemically. The filter is habitat, and a filter that is not cleaned will simply reseed the pool.
7. Keep the pump running continuously for several days, holding an elevated sanitiser residual throughout, and brush daily.
8. Expect to repeat. Because the organism survives inside plastics, a second treatment a week later is normal rather than a sign of failure.

Products, and what to expect from them

Ordinary algicides are usually disappointing against pink slime because they are formulated for plants. Products intended for bacterial slimes and biofilms are a better match. Whatever you use, it is a registered pesticide and the label directions carry legal force — the EPA's list of registered disinfectants is a reasonable orientation to how such products are evaluated and what the registration means.

Two cautions. Never combine products in a bucket or in the skimmer; some combinations of pool chemicals react violently. And do not exceed a label rate in the hope of speeding things up — the protective slime layer is a physical barrier, so more concentration does not substitute for brushing.

Telling it apart from other problems

If the pink material will not wipe off and is in the surface rather than on it, it is not slime — see metal staining instead.