Everything You Need To Know About Pool Algae

How many times have you come home after a holiday away only to find swathes of green algae floating in your pool? Or removed the pool cover in spring and discovered black spots on the walls and floors?  It can be heartbreaking, not just because you can’t use the pool straightway, but also because you know it’ll take hours of scrubbing and shocking to get your pool back to its former condition.

But it doesn’t have to be that way. By understanding what pool algae is, why it gets in your pool and how to recognise the first signs, you can minimise the damage and prevent recurrences in the future.

In this article, you’ll find everything you need to know about pool algae, like the three common types you’ll find in your pool, the risks they pose to your pool and family, and a step-by-step guide on how to eliminate them. Then we finish with frequently asked questions, like what to do to prevent algae, which pool types are susceptible to it, and how long it takes to make your pool algae-free again.

Ready to give pool algae the flick? Then let’s get stuck in!

What is pool algae?

Pool algae is a microscopic aquatic plant often found in backyard pools. It feeds on nutrients you often find in water, like nitrates and phosphates. While it can be a nuisance, algae is a natural part of our ecosystem and produces about 50 per cent of the oxygen we breathe, so it’s not all bad, but it can be unsightly and pose a health risk to pool owners if it gets out of control.

Algae spores are introduced into your pool by wind, dirt or rain. They can also come in via swimming costumes, dogs or toys that have been in rivers, oceans or ponds. Typically, spores are invisible to the naked eye, but when chlorine levels drop or your pH is unbalanced, they can turn into blooms and change the colour of your pool almost overnight.

While there are thousands of types of algae, there are three you’ll find in your backyard swimming pool:

Green pool algae

Green algae is the most common algae in residential pools. The colour is due to the presence of chlorophyll, which is necessary for photosynthesis. While it floats on the surface and has a slimy appearance, it’s the least harmful of the three and the easiest to kill – as long as you catch it early. Having said that, it multiplies at an exponential rate, particularly if your pool water is warm, low on sanitiser or poorly circulated.

Yellow or mustard pool algae

Yellow pool algae, also known as mustard pool algae, is more common in warm or humid areas. Often mistaken for pollen or rust, it can be found in darker areas of your pool, like corners or under stairs. While yellow pool algae contains chlorophyll, it also has beta-carotene, which is responsible for the colour. And because beta-carotene is an antioxidant, yellow pool algae is often resistant to chlorine (an oxidiser), making it difficult to control or eliminate.

Black pool algae or blue-green pool algae

While it’s often called algae, black pool algae is actually a bacteria called cyanobacteria. Instead of floating on your pool water or hiding in corners, it appears as a cluster of black spots on your pool wall, floor or skimmers. If left untreated, these spots can grow from the size of a small coin to a dinner plate.

Like other algae, black algae contains chlorophyll, but it also has phycobilin compounds, which are blue and red, giving the algae its characteristic blue-green colour. And because cyanobacteria can extend its roots into crevices and has a protective coating, it’s often harder to treat than green or yellow pool algae.

Note: You may have heard some people talk about pink algae. This is usually seen in tropical areas or fibreglass/vinyl liner pools. You’ll also find it anywhere water is present, like your toilet bowl or sink. Like black algae, it’s a type of bacteria that can appear around cracks, corners or ladders. It’s also slimy and resistant to chlorine, so you’ll need to follow the same steps as you would for yellow or black algae.

How does algae affect your pool?

No matter what type of algae you get, there are several ways it can wreak havoc on your pool. Here are some of them:

Breeds bacteria

While the algae itself isn’t harmful to humans, the bacteria that feed on it are. This can result in health issues like rashes, fever, diarrhea and eye infections. Infestations can also attract mosquitos (they love eating algae and laying their eggs in it), which can turn your splash zone into a mosquito habitat!

Erodes pool shell

Algae can increase your pool’s pH, which can gradually erode your pool’s surface or result in stubborn calcium build-up. Plus, algae itself can stain your shell, liner or pool equipment. This is particularly true of black algae, which has long filaments and burrows deep into the surfaces of your pool, making it difficult to remove.

Clogs your filter

Algae is sticky and grows in clumps, which means it can get caught in your filter or skimmer. This prevents proper circulation and filtration, making your algae problem worse. What’s more, even if you clean or backwash your filter, spores can cling to your equipment and multiply.

Affects visibility

A green pool can pose a risk to swimmers. Poor visibility means that you can misjudge distances and injure yourself against steps or hard toys. It’s also harder to see if someone in the water is in distress, or to find something that’s fallen in the pool.

Makes your pool slippery

Green algae is slimy, which means it can make steps, ladders and pool surfaces slippery. This can lead to injury and falls.

How do you treat pool algae?

Now that you know why algae is bad for your pool, let’s look at what you can do to get rid of it. The process for eliminating all types of pool algae is similar – the only difference is that yellow and black algae take longer to eradicate.

Skim your pool

First, remove any environmental debris from your pool. Use a net to skim leaves and branches floating on the surface – and don’t forget to empty the skimmers and pump basket. Organic matter can make your sanitiser less effective, so it’s best to remove as much of it as possible so your pool shock can do its job properly.

Scrub the walls and floors

Using a pool brush, scrub the walls and floors of the pool. Pay attention to corners, steps and ladders. This will help dislodge any algae that may be clinging to these areas. Scrubbing will make your pool water cloudy, but this is normal. Your vacuum will remove the debris in the next step.

Top tip: If you have a concrete pool, use a wire or stiff brush. If you have a fibreglass or vinyl liner pool, opt for a brush with nylon bristles. Using the wrong brush could result in damage to your pool surface. Not only can this affect its longevity, but it’ll make it easy for algae to take root!

Vacuum the pool

Now it’s time to get rid of all the algae or debris you dislodged with your brush. The best way to do this is with a manual vacuum. While this is a strenuous process, it helps you remove everything without contaminating your filtration system. (Algae spores can get caught in the filter and then circulate back in the water, undoing all your hard work.)

Depending on how much debris or algae is in your pool, you may lose a bit of water when you vacuum your pool, so after you finish, top up your pool water until it’s halfway up your skimmer box.

Check your water balance

Before adding shock, it’s a good idea to test your pH and alkalinity levels with a test kit (or take a sample to the pool shop). High pH or low alkalinity levels can make your pool shock ineffective, so it’s important to correct these levels before proceeding to the next step.

Shock your pool

Now it’s time to shock your pool. With your pump running, pour the recommended amount of pool shock for your pool size into the water. (You can even put your brush and vacuum head at the shallow end of the pool so they get a good clean too.) This will hyperchlorinate your pool to kill off algae, bacteria and other nasties. If you’ve got green algae, shock it twice. Yellow and black algae will require multiple shocks, at least until the water begins to clear.

Liquid chlorine can be added directly to the pool. If you’re using powdered shock, add it to a bucket of water and then mix before pouring it into the pool. Wear gloves and goggles throughout this process as chlorine is a caustic chemical and can cause skin injury.

Top tip: Try not to shock your pool during the day. The sunlight will reduce the effectiveness of your sanitiser. Start in the late afternoon when the UV reading is low.

Circulate the water

Run your filter for at least eight hours or overnight to circulate the shock throughout the pool. Twenty-four hours is optimal if you have yellow or black algae. If the water is still green or cloudy, you’ll need to shock again and circulate the water for another 8–24 hours until your water runs clear. When it does, test the water again and rebalance if necessary. Shocking can sometimes throw your water chemistry out of whack.

Backwash or clean your filter

During circulation, your filter is likely to have captured a lot of debris or bacteria. This needs to be removed to avoid recontamination, so backwash if you’ve got a media filter or give your filter cartridge a deep clean. You can perform this at the end or a few times during filtration if your pool was particularly algae-ridden.

Use a flocculant (optional)

If your algae problem is in the early stages, you may be able to remove it from your pool with a flocculant. The chemicals in the flocculant bind with the algae particles and help you vacuum them out of the pool. This is best done with a manual vacuum to bypass the filtration system. Just turn off the pool pump to recycle/recirculate and add the recommended amount of flocculant to the pool water. Allow it to circulate for a couple of hours, then turn off the pump and let it sit overnight.

In the morning, turn the multiport valve to waste and vacuum the debris from the bottom of your pool. If it gets cloudy, wait for it to settle, then start again. You can follow this with pool shock to remove any lingering algae spores from the water, then filter until the water runs clear.

Use an algaecide (optional)

If your algae problem is mild, using an algaecide alone may eliminate the problem. Just make sure you use one that’s appropriate for the type of algae in your pool and follow the instructions on the bottle. A metal-free, non-foaming algaecide that kills most types is Zodiac Supreme Pool Algaecide.

On the other hand, if you’ve got black algae, it may require treatment with Zodiac Black Spot Remover (ideally before brushing the walls and adding shock). Before you do this, make sure that you’ve actually got black algae and not a metal stain; they can look very similar. A simple test is to brush the stain. If it starts to come off, it’s algae. If it doesn’t, it could be a metal stain. In that case, it’s best to tackle the problem with a metal stain remover.

Top tip: If you’ve tried everything and you still can’t get rid of your algae problem, you may have a severe infestation on your hands. In that case, contact a pool professional for expert advice. They may need to drain your pool and acid wash/resurface the pool to tackle the problem. It’s best not to attempt this yourself as you may accidentally pop or damage the shell.

Can you prevent pool algae?

Algae spores are everywhere, but there are ways to minimise their impact on your pool water. While preventative measures take time, they’re easier and cheaper than fixing a nasty algae problem further down the line.

How to prevent pool algae

  • Keep your water balanced. Checking your water balance weekly is a great way to keep algae at bay. It means that your chlorine levels remain at 1–3 ppm, which is optimal for killing bacteria and algae. And you can identify and treat phosphate levels that may feed algae.
  • Circulate your pool water. Make sure you run your pump for at least 8 hours a day to ensure that debris is filtered out of your pool. Follow this with a backwash or filter clean when your pressure gauge is 8–10 PSI above the normal level.
  • Shock weekly. To nip algae in the bud, it’s best to shock your pool weekly during peak swimming season. This should be part of your weekly pool maintenance routine. It’ll help you catch anything that your sanitiser has missed. Heavy use, like pool parties, may require additional shocking.
  • Remove phosphates. As mentioned earlier, algae loves feasting on phosphates, so it’s best to keep levels under control. Environmental debris, sweat and skin products can all add phosphates to your pool water, so test regularly and use Zodiac Phosphate Remover if required.
  • Use algaecide. Even if you don’t have any signs of algae, monthly use of a metal-free algaecide like Zodiac Pool Algaecide can kill spores and prevent blooms from appearing or multiplying. Factors like heat, sunlight, rain and heavy use can all reduce the effectiveness of chlorine, predisposing your pool to algae, so this is a great preventative measure.
  • Lower the temperature of your pool. There are a few reasons why lower temperatures are better for your pool. First, heat can deplete chlorine and affect pH, allowing algae to take hold. Second, algae thrives in hot conditions, so the warmer the water, the faster it can multiply. That’s why winterising your pool can promote the dreaded bloom, particularly if you’re not circulating your pool water as often, or if haven’t treated the water with a preventative algaecide.

Are some pool types more prone to algae?

In general, concrete pools are more susceptible to algal growth. This is because the surface is porous and algae can grow in the grooves and become difficult to remove. As a result, concrete pools require more aggressive treatment and can take up to a week to become algae-free.

Vinyl liner pools are more resistant, but algae can still grow in seams or tears. Fibreglass pools, however, a less prone because they have a smooth surface. If algae does develop, it’ll appear in the water, not on the walls, which makes elimination relatively easy.

Do I need to use an algaecide in winter?

If you’re closing your pool for winter, it’s a good idea to use a long-life or winterising algaecide to keep algae at bay. This is different to regular algaecide as it’s designed to last for much longer. Even if you’re only running your pump for a few hours a day and your maintenance is a bit sporadic, it’ll provide backup protection against algae. After all, there’s nothing worse than pulling back your blanket in spring and finding a green pool!

How long does it take to get rid of algae?

This depends on a range of factors, like the pool surface, the type of algae you have and how long it’s been there. Understandably, small traces of algae can be nipped in the bud with pool shock. Yellow and black algae, due to their resistance to chlorine, may take longer. In general, fibreglass pools will be faster to treat and take about 24–48 hours. A concrete pool could take up to a week, particularly if the algae is widespread and has entered gaps or cracks in the surface.

Final thoughts

No one enjoys finding algae in their pool – and getting rid of it is no easy feat. The best course of action is to take preventative measures, like keeping your pool chemistry balanced, running your filtration system for at least 8–10 hours a day, shocking your pool weekly, using algaecide, avoiding high water temperatures and reducing phosphates. These actions should make your pool unfriendly to algae – green, yellow or otherwise.

If the blooms have made themselves at home, follow our step-by-step guide above, and don’t use the pool until the blooms or stains are completely gone and the water is balanced and running clear. For those who want to reduce their risk, consider buying or upgrading to a fibreglass pool. The smooth, slippery surface makes it difficult for algae to gain a foothold, and even if you do get some spores or blooms, they can be removed a lot faster.

So don’t wait until your pool goes green! Contact one of our approved dealers to get everything you need to make your backyard pool algae-free all year round!