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You scrub the pool, the water looks clear, and then a few days later the dusty yellow-brown patches are back on the shady side. If that sounds familiar, you are almost certainly dealing with mustard algae in pool water, and it is one of the most stubborn problems a pool owner can meet. The good news is that it is completely treatable, and once you understand how it behaves, you can get rid of it for good.
At HiLife Pools we design and build residential swimming pools across the UK, and we speak to owners every week who are caught out by this one. This guide explains what mustard algae is, why it keeps coming back, how to remove it step by step and how to stop it returning.
What is mustard algae?
Mustard algae, sometimes called yellow algae, is a type of algae that forms a yellow, brown or rust-coloured film on the surface of a pool. It tends to look like sand or dust that has settled on the walls and floor, which is why many people mistake it for dirt or pollen and try to sweep or vacuum it away.
It is different from the green algae that turns a whole pool pea-soup colour. Mustard algae usually clings to surfaces rather than floating in the water, so the water itself may look reasonably clear while the walls and steps look grubby. You can find out more about the more familiar kind in our guide to pool algae.
How to recognise mustard algae in your pool
It helps to know the signs, because treating the wrong problem wastes time and chemicals. Look out for the following.
- Yellow, tan or brownish patches, usually on shaded walls, steps, corners and the floor.
- A powdery film that brushes off easily but reappears within a day or two.
- Patches that return even after normal chlorine levels have been kept up.
- Growth on ladders, pool toys, covers and anything else that stays in the water.
- A slightly slimy feel to the surface rather than a gritty one.
A quick test is to brush the patch. Dirt and pollen tend to disperse into the water and then settle somewhere else. Mustard algae clouds up and then reforms on the same surface.
Why does mustard algae keep coming back?
The main reason is that it is more resistant to chlorine than most other algae. A normal chlorine level that keeps green algae away may simply knock mustard algae back for a short time. If any of it survives, it quietly regrows.
It also hides. Spores can live in places the water does not circulate well, such as behind ladders, in corners, in filter housings, in the skimmer and on anything you put into the pool. They can be carried in on swimwear, towels, toys and even on garden equipment. Treat the pool, but leave the swimsuit or the pool float untreated, and you can reinfect the water within days.
Poor circulation, low sanitiser levels, an unstable pH, warm water and sunshine all help it along. A pool that has been left under a cover or uncirculated for a while is a prime target.
How to get rid of mustard algae in pool water, step by step
Removing it needs a methodical approach. Cutting corners is the most common reason for it returning. This is the process we recommend.
Step 1: Test and balance the water
Start by testing the pH, the alkalinity and the sanitiser level. Chlorine works best when the pH is in the correct range, which for most pools is between 7.2 and 7.6. If your pH is too high, the chlorine will not be able to do its job properly, however much you add. Our guide on how to raise the pH in a pool covers the opposite problem, and the principles are the same: test first, then adjust.
Step 2: Brush every surface
Using a stiff pool brush, scrub the walls, floor, steps, corners and any fittings. Brushing breaks up the algae so that the chemicals can reach it. Take your time with the shady areas, which is where mustard algae likes to live.
Step 3: Shock the pool
Mustard algae needs a much stronger dose than normal. You will need to shock the pool with a chlorine product, often using a higher dose than you would for a routine shock. Follow the instructions on the product you are using carefully. If you are not sure how, our guide to how to shock a pool takes you through the basics.
Keep the pump running while the shock treatment works. The water should be circulated continuously, and the chlorine level should be kept up for the length of time the product recommends.
Step 4: Treat everything that has been in the water
This step is the one most people miss. Wash swimsuits, towels and toys in hot water, soak pool equipment in a strong chlorine solution and rinse it properly, and clean the ladders and any other equipment. If you do not, you may well bring the algae straight back.
Step 5: Clean the filter
The filter can hold mustard algae and release it back into the pool. Backwash a sand filter, and clean or replace the cartridge. Our article on how to clean a pool filter shows you how.
Step 6: Brush and vacuum again
Once the algae has died, it will look pale grey or white. Brush it off and vacuum it out, and then check your water chemistry again. You may need a second treatment if you can still see yellow patches.
Step 7: Keep monitoring
Test the water daily for the first week, keep the sanitiser at the right level and keep brushing the shady areas. If you see the first sign of yellow returning, act at once. A small patch is far easier to treat than a large one.
How long does it take to clear?
For most owners, mustard algae in pool water is cleared with patience and a methodical approach. A light case can improve within a day or two of a thorough treatment. A heavy or long-running infestation can take a week or more, and sometimes needs a second round. The cause is usually something being missed, such as equipment that was not cleaned or a filter that was not treated. If the problem keeps returning after two proper treatments, the next step is to look at the circulation and the system.
How to prevent mustard algae from coming back
Once the pool is clear, a few habits will keep it that way.
- Keep the water balanced. Check pH, alkalinity and sanitiser levels regularly, and adjust them promptly.
- Brush weekly. Make a habit of brushing the walls, steps and corners, particularly the shady ones.
- Run the pump long enough. Good circulation stops algae from settling and spreads the sanitiser evenly.
- Clean the filter. A neglected filter is a perfect home for algae.
- Rinse anything new. Toys, floats and swimwear from other pools or the sea should be rinsed before they go in.
- Follow a routine. A simple checklist makes it easy to stay on top of things, and our pool maintenance checklist is a good place to start.
Mustard algae in pool water compared with green and black algae
Telling the three common types apart is useful, because each needs a slightly different approach. Green algae is the one most owners know. It turns the water cloudy and green, spreads quickly and usually responds well to a normal shock treatment and a good brush.
Black algae forms dark spots, often on rough surfaces such as plaster or grout, and has deep roots that make it hard to remove. It needs aggressive brushing and a stronger chemical treatment. Mustard algae sits in between. It does not usually colour the water, it clings to surfaces like a film, and it can survive normal chlorine levels. If you see yellow patches that keep returning, think mustard algae first.
It is common for more than one type to be present at the same time, particularly in a pool that has been neglected for a while. Treat the most resistant one and the others will usually be dealt with along the way.
When is mustard algae most likely to appear?
It can appear at any time of year, but it is most common in spring and summer. As the water warms up and the sun gets stronger, algae of all sorts find the conditions they like. Pools that have been closed for winter, or that have been under a cover with the water left unattended, are particularly prone to it when they are opened.
Heavy rain, a hot spell, a large number of swimmers or a period when the pump was off can all tip the balance. If your pool has a shady corner that does not get much sun or circulation, that is where it is most likely to start. If you are opening up for the season, our pool opening checklist explains what to check before the first swim.
Treating pools with covers, salt systems and heat pumps
The treatment is essentially the same whatever your set-up, but there are a few details worth knowing. If you have an automatic cover, brush and clean the underside and the track as well, because algae can hide there. If you use a salt chlorinator, you may need to boost the chlorine output or add a manual shock to reach the levels you need, and you should check the manufacturer’s advice for your system.
A heated pool is warmer for longer, which gives algae more opportunity to grow, so good circulation and regular testing matter even more. Whatever the equipment, the basics stay the same: balanced water, thorough brushing and clean kit.
Is mustard algae dangerous?
It is not generally thought to be harmful in the way some bacteria are, but it is not something you want in your pool. It can make surfaces slippery, make the water look unappealing and use up your sanitiser. Many people also prefer not to swim in a pool that has visible algae growth. If you are in any doubt, keep swimmers out until the water is clear and tested.
Does the design of the pool make a difference?
It can. Pools with awkward corners, poor circulation or dead spots tend to be more prone to algae. Good design means that the water flows through the whole pool, the skimmers and returns are positioned well and every surface can be reached easily. It is one reason we put so much thought into the layout, the plant room and the circulation when we design a new pool.
If you are planning a new pool, or you are thinking about upgrading an old one, it is worth asking about how the water will circulate and how easy it will be to look after. A pool that is simple to maintain is a pool you will actually enjoy using.
Common questions about mustard algae in pools
Can mustard algae live outside the pool?
Yes, it can survive on damp surfaces such as swimsuits, toys, pool covers and equipment for some time, which is why it is so easy to spread it back into clean water.
Will it go away by itself?
Unfortunately not. Without proper treatment, it usually spreads and gets worse as the water warms up.
Do I need to drain the pool?
In nearly all cases, no. A thorough, methodical treatment is normally enough. Draining is rarely needed unless the water is in poor condition for other reasons.
Can I swim during treatment?
No. Wait until the chlorine level has come down to a safe level for swimming, and test it before anyone gets in.
Speak to HiLife Pools
If you would like help keeping your pool in top condition, or you are thinking about building a new one that is designed to be easy to look after, we would be delighted to talk. Get in touch with the HiLife team for a friendly, no-pressure chat, or have a look around our home page to see our work.
