In this article, we show you how to spot and treat mustard algae and other resistant algae strains in your swimming pool.
Thankfully this type of algae does not occur regularly, however, it can be quite stubborn to treat as it is resistant to conventional chlorine treatment.
Pool owners can often mistake mustard algae for dirt, as their pool cleaner fails to collect it. This is because mustard algae cannot be collected by a pool cleaner. It will simply pass through the cleaner and filter and return shortly after. This problem must be treated with chemicals.
Signs of Mustard Algae
Mustard algae presents differently from regular algae problems. Here's what to look for:
Looks like fine sand
Yellow or brown deposits settle on the pool floor and walls
Not slimy
Unlike black spot, mustard algae has a dry, dusty texture to the touch
Brushes off easily — and comes back
It disappears with a brush but regrows quickly without chemical treatment
Spreads fast
Appears quickly on walls, steps, pool toys, cleaners and other equipment
Clouds the water
Causes cloudy water and can leave staining on pool surfaces and liners
Is Mustard Algae Harmful to Humans?
Mustard algae itself does not pose a direct risk to humans, however it devours chlorine, making it difficult to maintain healthy sanitation levels in your pool water. This increases the risk of harmful bacteria growth. Bacteria such as E. coli can thrive in an environment overrun with mustard algae.
How to Get Rid of Mustard Algae
Start treatment as soon as you notice fine sand or dirt appearing on the pool floor or other surfaces early treatment will have a faster outcome. The following treatment is broad-spectrum and recommended for all mutant algal strains.
Vacuum to waste
Vacuum your pool to waste to remove as much algae as possible before treatment begins.
Clean your filter
Backwash or hose cartridges thoroughly. Algae will thrive inside your filter and reinfect the pool if this step is skipped.
Adjust pH to 7.2-7.6
If adjustment is needed, filter for 4 hours before moving to the next step.
Add a clarifier cube
Drop 1 clarifier cube into the skimmer box.
Add liquid chlorine
Up to 20kL
10L
20kL – 55kL
15L
55kL – 70kL
20L
70kL – 100kL
30L
Brush vigorously
Brush all pool walls thoroughly. Keeping the algae in suspension is the most effective way to eliminate it don't skip this step.
Run filter 24/7 until clear
Keep the filter running continuously until water clarity returns typically 2-4 days. Backwash and brush once daily throughout.
Rebalance your water
Test regularly and rebalance as needed until levels are stable.
Fibreglass & Vinyl
~1 week
with correct treatment
Concrete & Pebble
Up to 3 weeks
continuous brushing required
Prevention
Treating stubborn algae is time-consuming and costly, so prevention is always preferable. Test your pool regularly particularly in summer and during periods of heavy use as this is when chlorine levels are most likely to drop.
Test regularly through summer
Particularly during periods of heavy use when chlorine burns off fastest.
Top up chlorine before levels drop
Don't wait until you have a problem apply a shock treatment proactively during hot weather or high bather load.
Catch it early
Treatment started at first signs of mustard algae is significantly faster and cheaper than treating a full outbreak.
