The White Stuff Floating in Your Pool Is Mold — Here’s How to Get Rid of It

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If you see whitish deposits on the surface of your pool water that look like bits of tissue paper or mucous, your pool has a case of white water mold. It's often accompanied by a pinkish buildup known as pink slime, which collects on the pool walls, especially around ladders and other fixtures. White water mold is caused by ‌Saprolegnia‌ spp.‌,‌ a funguslike organism that belongs to a class called Oomycota that also includes downy mildews. Pink slime is a bacterium — ‌Serratia marcescens‌ — that is also commonly found on toilet bowls, shower walls, and other places in bathrooms. Both are common and difficult to keep away.

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White water mold and pink slime are resistant to halogen-based sanitizers, such as chlorine and bromine, but you can still eradicate them by triple- or quadruple-shocking the water with chlorine shock and meticulously cleaning the pool and filter. The eradication won't happen overnight; it may take several days.

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Here's how to get rid of white water mold in your pool so you can safely and cleanly swim all summer long.

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What Is White Water Mold?

Scientists used to classify white water mold (‌Saprolegnia‌ spp.) as a fungus, but research has shown that the class of organisms to which they belong, Oomycota, differs from fungi in significant ways. The danger to swimmers is minimal because the organisms die as soon as you get out of the water and dry off, but unfortunately it's difficult to kill while it's in the pool. This is due to its chitin-enclosed cell walls, which protect it from pool sanitizers. The cell walls aren't completely impervious, though, and very high levels of chlorine can kill them. A strategy for eliminating white water mold involves physically removing as much as you can and shocking the pool to take care of the rest.

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What Is Pink Slime?

Serratia marcescens‌ (or pink mold), which is often seen alongside white water mold, was first identified in 1819. It contains a pigment called prodigiosin that gives it its red coloration, and because it was thought to be harmless, it has been used as a tracer organism in medical and dental practice. Today, health professionals now know that it can cause urinary tract infections, wound infections, and pneumonia in a small percentage of people with compromised immune systems. The procedure for getting rid of it is similar to that for eradicating white mold. It isn't as chlorine-resistant as white water mold, but what makes it difficult to control is that any small amount you fail to remove from the pool will grow into a new colony.

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Things You'll Need

How to Get Rid of White Water Mold

If white water mold is growing in a chlorine pool or a bromine hot tub, the procedure for eradicating it consists of triple- or quadruple-shocking the water and then meticulously brushing the sides of the pool or hot tub, vacuuming the bottom and cleaning the filter. If you use biguanide in your pool, which is a gentler sanitizer than chlorine, use an oxidizer instead of chlorine shock.

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1. Clean the Filter

Backwash your sand or DE filter. If you have a cartridge filter, take it out and wash it down with a garden hose. You need a clean pool filter to complete the sanitization process.

2. Balance the pH

The pool shock won't be effective unless the pH of the water is in a range from 7.4 to 7.6, with 7.5 being ideal. Raise the pH using sodium bicarbonate or soda ash and lower it using muriatic acid or dry acid.

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3. Shock the Water

To get rid of white water mold, you need to add three to four times the amount of shock you normally would. This means pouring in 3 to 4 pounds of shock per 10,000 gallons of water. The goal is to get the chlorine levels up to at least 5 ppm. It's best to add shock in the early evening so it has all night to work without being degraded by the sun.

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4. Brush the Pool, Filter the Water, and Brush Again

Use a stiff pool brush to brush down the sides and bottom of the pool. The goal is to remove as much of the biofilm as possible. When you're finished, run the pool pump overnight or longer, preferably 24 hours, to trap as much of the loose debris as possible in the filter. After you do this, brush the sides of the pool again because you don't want to leave even a tiny amount of mold behind, or it will grow right back. Let the debris settle on the bottom of the pool.

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5. Vacuum the Pool

Set your pool vacuum to go to waste to prevent the water from running through the pool filter and manually vacuum the bottom of the pool. This may cause the water level to drop significantly, so have a garden hose handy so you can add more water.

6. Clean the Filter Again

This time, do a deep clean of the filter. Backwash your sand or DE filter and then use a filter cleaning product to clean the grid. If you have a cartridge filter, immerse it in cleaning solution for the time recommended on the container.

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7. Test and Balance the Water

Test the water using test strips. You want the chlorine levels to drop to 3 ppm, the pH to hold steady between 7.4 and 7.6, the total alkalinity in a range from 80 to 120 ppm, and the calcium hardness from 125 to 150 ppm. If you use a cyanuric acid as a chlorine stabilizer, you want the level between 30 and 50 ppm.

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Tip

Keep an eye on the pool for the next week or so to make sure the mold doesn't reappear. It's a good idea to brush the pool surfaces and vacuum every other day and to run the pump as often as possible.

How to Get Rid of Pink Slime

The procedure for getting rid of pink slime is similar to that for getting rid of white water mold, but there's a difference in the pool chemicals you use and how you administer them. In particular, you need a product designed to kill the bacteria that cause pink slime, and since this is often called pink algae, the product may be labeled as an algaecide. Read the label carefully to be sure it's supposed to be used for pink algae. Pink Pool Treat is one such product.

Here's how to get rid of pink slime in your pool:

  1. Clean the filter and balance the water, just as you would when treating white water mold.
  2. Then turn off the pump, brush down the sides and bottom of the pool, and pour in the algaecide before triple- or quadruple-shocking the water.
  3. After letting the pool sit undisturbed overnight, you vacuum the bottom, clean the filter again, and turn on the pump.

To prevent the bacteria from growing back, you'll need an unusually clean pool. To achieve this, keep the free chlorine level at 5 ppm for a week. This level is too high for comfortable swimming, so plan on devoting this week to pool maintenance instead of recreation. Test the water daily, maintain the pH between 7.4 and 7.6, and add chlorine as needed to keep the concentration high.

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