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Yuck! What’s that?

A pink slime has appeared in your pool seemingly overnight and is now clouding the water. What’s the trigger? Wild animals, contaminated fill water, or was it just a prank by the neighborhood kids?

In fact, your disinfectant is the direct cause of the pool turning red. But how can that be? Surely the red algae should be killed by the chlorine?

The answer is: yes and no

It is true that you can kill algae and other foreign matter with chlorine. However, the supposed red algae in the pool are not algae at all. They are bacteria that form a biofilm on the walls and bottom of the pool.

What is red algae?

When there is red coloration or buildup on the walls or bottom of the pool, people often mistakenly refer to it as red algae in the pool. The heel, called “pink slime” in English, is actually a biofilm consisting of bacteria.

Everyone knows the principle of a biofilm from teeth. The coating that forms on the teeth during the day is nothing other than a biofilm. Our mouth is full of microorganisms. The plaque bacteria deposit on our teeth as bacterial plaque.

In general, the term “biofilm” means bacteria that are deposited on objects. The bacteria in the mouth, just like the “red algae”, are harmless to human health.

The red algae – or rather the biofilm – can grow in any kind of pools. They settle on walls and the bottom of the pool. The causes of it can be too low dosed disinfectant (for example chlorine), adverse weather conditions or neglecting pool maintenance.

In the first stage, the biofilm is usually not reddish, but only colorless. If you notice that the floor and walls suddenly become slippery, this is a first warning sign. In another manifestation, thread-like formations may also appear.

The red coloration often only occurs when you add more chlorine (e.g. in the form of chlorine tablets) or another disinfectant to the pool water again, because then the bacteria die and leave behind their characteristic pink color.

Pool owners are often irritated because red algae supposedly forms in the pool when you actually disinfect the pool. However, the colorless biofilm was already in the water all along and now only became visible when the disinfectant was added.

The discoloration occurs when using all common pool disinfectants, as the phenomenon is based on the fact that the bacteria are killed and a pinkish tint is naturally created.

The best way to solve the problem with biofilm, or red algae in the pool, is to firstly remove the biofilm from the pool and then take effective preventative measures so that biofilm can no longer form in your pool.

Removing red algae from the pool

Removing red algae from your pool requires more than just disinfectants. Even in our plaque example, adding an agent such as a mouthwash is not enough to keep your mouth clean.

For plaque, you need a toothbrush, and even with the pool, it’s a matter of mechanically removing the thin film. Daily practice brushing your teeth has already made you a real expert at removing a biofilm.

With the following step-by-step instructions, we build on this, so to speak, and show you how to remove a biofilm or how to remove red algae from the pool. After that, we will give you more tips on how to prevent a biofilm from forming in the swimming pool in the first place.

Step 1: Test the pool water

Balanced pool chemistry is a necessary condition for the following steps.

Check the pool water with test strips or a test kit. The most important values here are chlorine content, pH and alkalinity.

Pool and Spa Test Strips - Quick
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Step 2: Correct the pool chemistry

Compare the measured values with these ideal values:

  • pH: 7.0 to 7.4
  • Total alkalinity: 80 to 150 ppm
  • Chlorine level: 2 to 3 ppm

If the measurements are all OK, you can move on to the next step. If your values are not in these ranges, you will need to adjust.

Unbalanced pool chemistry could inhibit the disinfecting action of chlorine. For example, a too high pH or too low alkalinity would negatively affect the action.

Correcting the pool chemistry is straightforward. You can raise or lower the pool pH with the two chemicals, pH-Plus and pH-Minus.

Clorox Pool&Spa 12105CLX pH Down, 5
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Total alkalinity (TA) is less easy to adjust than pH. There are chemical agents for increasing the alkalinity, but lowering the concentration can only be achieved by (partially) replacing the water or by adding pH minus to the center of the pool in one batch.

Always adjust the alkalinity first, before you worry about the pH and then the chlorine level.

The chlorine content degrades over time. For this reason, you need to regularly add new chlorine granules or chlorine in tablet or powder form.

HTH 22008 Ultimate Mineral
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Step 3: Clean the pool

Brush down the walls and bottom of the pool. Pay attention to the corners and other hard-to-reach places (such as behind the ladder).

Use as sturdy a pool brush as possible. For pools made of plaster or concrete you should choose a pool brush with steel bristles, for PVC pools you need a softer model, for example with nylon bristles.

We are convinced by this pool brush with nylon bristles. For inflatable pools, the brush is very suitable:

POOLAZA Pool Brush, 17.5" Pool
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Use a pool vacuum (such as a manual pool vacuum) to vacuum off the scraped bacteria. To use a pool vacuum, you must set the valve on your filtration system to waste (“Waste”). Keep in mind that the pump must be off when you want to switch the valve.

If the valve is set to waste, the vacuumed pool water will be piped directly out of the pool. When vacuuming using a garden hose, make sure there is a permanent flow of water so that the water level does not drop too low.

Swimline Weighted Flex Vacuum Head,
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Step 4: Backwash

Bacteria just love the PVC pipes of your filtration system. Let the filter backwash for at least 5 minutes to get the bacteria out of the system.

Step 5: Triple shock chlorination

Now you’re ready for a shock chlorination. Add pool shocker after dark with circulation running. Using the package information, determine the recommended dose for your pool and then multiply that value by three to have a dosage that really kills the bacteria.

Note: Use only chemicals specifically made for shock chlorination. It is important that you use unstabilized chlorine, as it will quickly break down in the sunshine.

HTH Pool Care Shock Advanced,
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Step 6: Filter for 24 hours with filter cleaner

Add filter cleaner to the filter and then run it continuously for 24 hours. After that, perform a backwash again.

Step 7: Test Numero 2

One day after shock chlorination, you should perform another water test. Use test strips or a test kit again to do this. Make sure that the pH, alkalinity, and chlorine levels are connecting in the correct range.

It is possible that the concentration of chlorine is low. If the biofilm has not been completely removed, the chlorine will continue to be consumed during disinfection.

An indication that there is still biofilm or red algae in the pool is a chlorine level below 1.5 ppm. In this case, perform another shock chlorination and add more chlorine granules to the pool.

Tips to prevent red algae in the pool.

The supposed red algae in the pool does not have to happen. With a few tricks, you can effectively prevent a biofilm from forming in advance.

  • A common trigger for the bacteria to settle in the pool water is that when the pool owner goes on vacation, no one takes care of pool maintenance.
  • Over time, chlorine levels drop, making your pool an ideal home for bacteria. In the best case scenario, you have someone taking care of the pool chemistry. If that’s not the case, you should at least vigorously add chlorine granules to the water before you leave.
  • Always maintain a chlorine level of 2 to 3 ppm. If the sanitizer level is too low, the bacteria will not be killed.
  • Brush and vacuum the walls and bottom of the pool regularly to prevent biofilm buildup. It’s best to use an automatic pool cleaner or a fully automatic pool robot to make your job easier and keep the pool sparkling clean at all times.
Dolphin Nautilus CC Plus Robotic
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Now you know: the pink slippery layer in your pool is not algae, but actually a deposit of bacteria.

But whether it’s biofilm or red algae, the main thing is that you know how to remove these foreign bodies and keep them out effectively.

Larry has been a true water rat since childhood. Pure pleasure turned into a passion. That's why he is the first point of contact for friends and acquaintances when it comes to pool-related problems. He is an integral part of the PoolHandbook editorial team.