Black Algae and Mustard Algae: How to Get Rid of Them
Most pool owners know green algae: the water goes hazy, then green, and a proper shock and filtering bring it back. Mustard algae and black algae are different. They cling to the surface rather than floating in the water, they survive chlorine levels that kill green algae, and they come back if the treatment is not thorough. The good news is that both can be cleared without draining the pool in most cases. The first step is being sure what you are dealing with.
Identify what you have
Green algae
Green algae floats in the water and grows on walls and steps as a slippery green film. The water itself turns cloudy or green. It brushes off easily and responds well to shocking and filtering. If this is your problem, our post on why your pool turns green and how to fix it covers it in full.
Mustard algae
Mustard algae is yellow, yellow-brown or yellow-green, and often looks like pollen, dust or sand on the walls. It usually appears first on the shady side of the pool and in corners and steps where circulation is poor. The water can still look clear.
The telling sign is that it brushes off very easily, into a cloud, and then settles back in the same places a day or two later. It also survives on anything that has been in the water: costumes, toys, pool noodles, the automatic cleaner, brushes and nets. That is why it keeps coming back.
Black algae
Black algae appears as small, dark blue-black or black spots, often with raised edges, from pinhead size to a few centimetres across. It prefers rough, porous surfaces such as older plaster and marbelite, and is common in shady spots, cracks and around fittings.
It is the hardest to shift. Each spot has a protective outer layer that shields it from chlorine, and it roots into the pores of the surface. Brushing with a normal nylon brush barely touches it.
Algae or a stain?
Not every dark or discoloured patch is algae. Metal stains from iron or copper, and tannin stains from leaves left lying on the floor, can look similar. Treating a stain with algaecide wastes money, and treating algae as a stain lets it spread.
A quick check on a plaster or marbelite pool: put a chlorine tablet in an old sock, hold it against the spot for half a minute or so, and look again. If the spot lightens or fades, it is often algae or an organic stain. If nothing changes, it may be a metal stain, which needs a different treatment. Wear gloves, and do not do this on fibreglass or vinyl, which can bleach or mark. This is a guide rather than a definitive test, and the pattern matters too: black algae grows as spots with defined edges, while metal stains tend to be broader and more even.
How to treat mustard and black algae
The steps are similar for both. Black algae simply needs more of each, and more patience.
1. Balance the water first
Test and correct total alkalinity to 80–120 ppm and pH to 7.2–7.6 before adding chlorine. At high pH, chlorine works much less well, and much of your shock is wasted. Also check stabiliser (cyanuric acid). If it is well above 50 ppm, chlorine struggles to kill anything, and the only practical way to bring stabiliser down is partially draining and refilling.
2. Brush hard
Brushing breaks the protective layer on black algae and lifts mustard algae into the water where the chlorine can reach it.
- For mustard algae, a normal pool brush is enough, but brush every surface, including steps, benches and behind the ladder.
- For black algae on plaster or marbelite, use a stainless-steel algae brush and scrub each spot hard until the top of it is broken.
- Never use a stainless-steel brush on fibreglass, vinyl or painted surfaces. It scratches and damages them. Use a nylon brush and more time instead.
3. Raise the chlorine or shock
Shock the pool with a dose sized to your pool's volume, following the label, and add it in the evening so the sun does not burn it off. Mustard and black algae need a higher chlorine level for longer than green algae, so keep it elevated for several days, testing daily and topping up.
On a plaster or marbelite pool, some people rub a chlorine tablet directly on individual black algae spots after brushing. It can help with a few isolated spots, but do not leave a tablet lying on the floor, which bleaches and damages the surface.
4. Use an algaecide made for the job
Once the chlorine is up, add an algaecide suitable for mustard or black algae, following the product label for dose and timing. Some algaecides should not be added at the same time as a shock, so read the instructions. Algaecide supports the treatment; it does not replace brushing and chlorine.
5. Clean everything that touched the water
This is the step people skip, and the reason mustard algae comes back.
- Wash costumes and towels that were used in the pool
- Soak or wipe down toys, noodles, floats and goggles in a mild chlorine solution
- Clean the automatic cleaner, its hoses and any robot's filter
- Soak brushes, nets and the leaf rake
- Rinse and clean the weir basket and pump basket
6. Filter and backwash
Run the pump for longer than usual throughout the treatment so dead algae is caught in the filter. Backwash when the pressure gauge reads roughly 50–70 kPa above its clean reading, and vacuum settled debris to waste rather than through the filter, so you are not recirculating it. If the water stays cloudy even with correct readings, the filter sand may be caked and due for replacement.
7. Repeat
Brush again every day for a week or more, keep the chlorine up, and look closely at the spots. Mustard algae usually gives way to a thorough first round. Black algae often needs two or three rounds before the spots stop reappearing.
When black algae will not go away
Black algae that has rooted deep into porous, rough or ageing plaster can survive everything above. The spots fade but return in the same places, because the roots are inside the surface.
At that point an acid wash is often the real answer. It removes a thin layer of plaster or marbelite and the algae roots with it, leaving a clean, smoother surface that is harder for spores to grip. Our pool acid wash service is R3,500 for a full wash, and explains why winter is the better time to drain a pool. Acid washing is not for fibreglass pools.
Preventing it from coming back
- Keep free chlorine at 1–3 ppm every day, not only after a problem
- Keep pH at 7.2–7.6 so the chlorine you add actually works
- Run the pump long enough for the full volume to pass through the filter daily, and longer in hot weather
- Brush the whole pool once a week, paying attention to the shady side, steps and corners
- Rinse costumes and toys after swimming in natural water such as dams, rivers or the sea before they go into your pool
- Keep stabiliser within 30–50 ppm; with the wrong chlorine product it can creep up over a season, which our guide to pool chlorine types explains
- Replace filter sand when it is worn, so dead algae is actually removed
When to get help
If the pool has also turned green, or you have been through several rounds and the spots keep coming back, it is worth having someone look at the surface, the water and the equipment together. Our green pool recovery is a fixed price by pool size, from R1,200 for pools up to 30,000 litres, and includes a check of the pump, filter and chlorinator so you know what let it grow.
WhatsApp us on 081 577 5051 with close-up photos of the spots and a wider photo of the pool, and tell us what the surface is. That is usually enough for us to tell you whether it looks like algae, a stain, or a surface that needs an acid wash.