06/02/2026
🟡 Why Mustard Algae Loves North Texas Pools (And How To Kick It Out) 🟡 (amended long version)
by Deep End Frank Disher
You walk outside one morning and notice a yellowish-brown dusting on the floor of your pool.
No big deal.
You brush it.
It disappears.
A few hours later it's back.
Now you're annoyed.
Welcome to the wonderful world of Mustard Algae... one of the most misunderstood and stubborn pool pests we deal with in North Texas.
FIRST, LET'S CLEAR UP A HUGE MYTH
You cannot prevent mustard algae from getting into your pool.
You simply can't.
It's everywhere.
🌧️ Rain brings it.
💨 Wind brings it.
🐿️ Critters bring it.
🍃 Leaves bring it.
👣 Swimmers bring it.
🚜 Dust storms bring it.
North Texas is basically a giant algae delivery service.
The goal isn't preventing it from entering your pool.
The goal is preventing it from getting established.
WHAT EXACTLY IS MUSTARD ALGAE?
Mustard algae is often called yellow algae, although sometimes it appears brownish depending on lighting, surface color, and the amount present.
It can appear as:
🟡 Yellow dust on the floor
🟤 Brownish dusting on steps or floor
🟡 Yellow patches on walls
🟤 Stubborn growth on tanning ledges
🟡 Spots hiding in spa footwells
Unlike green algae, mustard algae often prefers attaching itself to surfaces rather than floating freely in the water.
Most of the time when I find it in North Texas pools, it's attached to walls, tanning ledges, corners, light niches, and other low-circulation areas.
Many people mistake it for dirt, pollen, dust, or sand.
A QUICK TEST
If you brush it lightly and it immediately disappears:
It may be mustard algae.
If it settles back into exactly the same location over the next day or two:
It's probably mustard algae.
If it disappears and settles right back to the floor within minutes:
It's probably dirt or debris.
WHY IS MUSTARD ALGAE SO DIFFICULT TO KILL?
The biggest difference between mustard algae and green algae is that mustard algae belongs to a group called diatoms.
Think of a diatom as nature's microscopic armored vehicle.
Diatoms use silica to build a protective shell around themselves.
It's not literally living DE powder, but it's close enough for pool owners to understand why it's tougher.
Think of it this way:
👕 Green algae shows up wearing a T-shirt.
🦺 Mustard algae shows up wearing body armor.
That shell makes it harder for sanitizers to pe*****te and kill.
MYTH: MUSTARD ALGAE IS CHLORINE RESISTANT
Nope.
Not true.
The best algaecide ever invented is still chlorine.
Always has been.
Always will be.
Mustard algae is not immune to chlorine.
It simply requires:
🧪 Higher chlorine
🧪 Longer exposure
🧪 Better circulation
🧪 Better brushing
🧪 Better filtration
The problem isn't that chlorine doesn't work.
The problem is that many pools don't maintain enough chlorine long enough.
WHAT MUSTARD ALGAE LOVES
Mustard algae absolutely loves:
⚠️ Dead spots
⚠️ Poor circulation
⚠️ High pH
⚠️ High calcium environments
⚠️ Low sanitizer
⚠️ Warm water
If you've ever heard me preach about circulation, this is exactly why.
The biggest trouble spots I see:
🏖️ Tanning ledges
🛁 Spa footwells
💡 Behind pool lights
🛟 Under floats
🪑 Under tanning ledge loungers
🌊 Corners with poor circulation
If water isn't moving, algae is planning a family reunion there.
In my imagination, there is a little underwater nightclub called "The Dead Spot" where all the algae gather every night to party.
MUSTARD ALGAE AND BLACK ALGAE
This gets interesting.
Mustard algae can slowly contribute to surface deterioration.
As surfaces become rough, etched, or damaged, they become easier for other algae species to colonize.
Black algae often establishes itself in these damaged areas.
Research has shown mixed-species biofilms can develop where multiple algae species live together.
Imagine:
🚙 Black algae is the armored vehicle.
🦺 Mustard algae is riding inside wearing a bulletproof vest.
The biofilm protects the colony.
The diatom shell protects the individual organism.
That's one reason severe algae infestations become increasingly difficult to eliminate.
WHAT ABOUT PHOSPHATES?
Here comes the controversial part.
Phosphates do NOT make algae impossible to kill.
Chlorine kills algae.
Period.
However...
Phosphates are food.
Lots of food.
The more food available, the faster algae can reproduce whenever chlorine drops.
Can you maintain a beautiful pool with high phosphates?
Absolutely.
Do excessive phosphates make algae outbreaks harder to control?
Also yes.
MY BORATE SOAPBOX
You knew this was coming.
Borates at 40-50 ppm are one of my favorite pool additives.
Borates help:
✅ Slow algae reproduction
✅ Improve water feel
✅ Stabilize pH
✅ Reduce chlorine demand
✅ Improve water clarity
Can you still get algae?
Of course.
But when algae does appear, it is often easier to control and eradicate.
How To Kill Mustard Algae
STEP 1: VACUUM
Remove as much visible algae as possible.
Every algae cell you physically remove is one less cell chlorine must kill.
STEP 2: BRUSH EVERYTHING
And then brush it again.
And then brush it some more.
🧹 Walls
🧹 Steps
🧹 Tanning ledges
🧹 Spa footwells
🧹 Behind ladders
🧹 Around lights
You want to expose every cell to sanitizer.
STEP 3: CLEAN THE FILTER
This is huge.
Backwash.
Clean cartridges.
Clean DE grids.
Do not leave dead algae sitting in the filter system.
You want it gone.
STEP 4: RUN THE POOL CONTINUOUSLY
24 hours a day.
Until the algae is gone.
This is not the time to save a few dollars on electricity.
STEP 5: LOWER PH
Target:
📉 7.2-7.6
Lower pH improves chlorine effectiveness.
STEP 6: RAISE CHLORINE
For green algae:
15-20% of CYA is often sufficient.
For mustard algae:
30% of CYA may be required.
Example:
CYA = 50 ppm
FC target = 15 ppm
The secret isn't reaching 15 ppm.
The secret is maintaining 15 ppm.
SODIUM BROMIDE TREATMENTS
Many mustard algae products contain sodium bromide.
When chlorine oxidizes sodium bromide, it creates hypobromous acid.
Hypobromous acid is an extremely powerful sanitizer.
Advantages:
✅ Fast acting
✅ Effective on difficult algae
✅ Often works overnight
Disadvantages:
⚠️ Consumes large amounts of chlorine
⚠️ Sunlight destroys it rapidly
⚠️ Can create lingering chlorine demand problems
⚠️ Not ideal for routine use
I consider sodium bromide a special forces weapon.
Not daily maintenance.
Apply it once.
Use it correctly.
Move on.
COPPER-BASED ALGAECIDES
Do they work?
Absolutely.
Do they stain pools?
Absolutely.
Copper works because algae absorbs it.
The algae eventually accumulates enough copper that cellular processes fail and the organism dies.
The problem is that copper doesn't disappear afterward.
It remains in the water.
That's where staining problems begin.
If using copper:
🧪 Test for metals
🧪 Monitor water balance
🧪 Maintain proper LSI
🧪 Consider products such as CULator when metals approach 0.2 ppm or greater
Copper has a place.
But it does not replace good chemistry and maintenance.
Using copper instead of proper pool care is like spraying air freshener under the sink instead of removing the rotting trash.
AMMONIUM SULFATE TREATMENTS
Some mustard algae products contain ammonium sulfate.
These products intentionally create chloramines.
Yes.
The same chloramines everyone complains about.
Ironically, those chloramines can be effective algae killers when used according to directions.
They also create significant chlorine demand.
Always follow the label carefully.
The Biggest Mistake Pool Owners Make
They wait until they SEE algae.
By the time you see algae, millions of algae cells are already present.
Many algae species can double every few hours under ideal conditions.
That's why:
🌧️ Big rainstorms
🎉 Pool parties
🍃 Heavy debris loads
⚙️ Equipment failures
🔌 Pump shutdowns
should always be followed by aggressive chlorine management.
Even if the pool looks perfect.
The algae is already there.
It's just waiting for an opportunity.
The Bottom Line
Mustard algae is not magic.
It is not chlorine-proof.
It is not unbeatable.
It simply takes advantage of poor pool management.
The solution remains the same:
✅ Good circulation
✅ Good filtration
✅ Good brushing
✅ Good water balance
✅ Consistent chlorine
Do those five things and mustard algae becomes a nuisance instead of a nightmare.
And remember...
If water isn't moving...
The algae are probably down at "The Dead Spot" nightclub planning their next family reunion.
Keep the water circulating, keep the filter clean, keep the surfaces brushed, and keep chlorine where it belongs.
When circulation and maintenance stay consistent, mustard algae never gets the chance to settle in and turn your pool into its favorite hangout.
