Mustard Algae

nicolaallen

Member
May 24, 2021
5
Lewisville, Texas
I think we have mustard algae. We are new pool owners. Bought a house in October with a salt water pool. Just over 16,000 gallons. We have been getting the water checked weekly at our local pool store and following what they say to do. The water is actually nice and clear, but I notice a lot of yellowish brown sediment on the bottom that brushes away easily-seems very powdery. There is some staining on the walls, but we have been brushing regularly and it comes off fairly easily, but then comes right back. According to our water testing, we have had to add shock and muriatic acid weekly. (a bag of shock and a 1/4 gal acid) How can I find out for sure that it is mustard algae and what's the remedy?
 

JohnT

Admin
Mod Squad
TFP Expert
Apr 4, 2007
10,006
SW Indiana
Pool Size
21000
Surface
Vinyl
Chlorine
Liquid Chlorine
Welcome to TFP!

You don’t have mustard algae. What you have is bad test results. Pool stores know next to nothing about water chemistry and their testing is uniformly bad. Get your own test kit and you’ll likely find you aren’t maintaining enough chlorine and you have plain old everyday algae which is easy to deal with. We recommend either the Taylor K2006 or the TF-100 from tftestkits.net.
 
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YippeeSkippy

Mod Squad
LifeTime Supporter
Jan 17, 2012
17,083
Evans, Georgia
Nicola, I can assure you that is not mustard algae. That may be dead algae, or pollen but its hard to know by your description.
First thing I will tell you is that we do not go to pool stores for testing. They're wrong as often as right. You should protect your investment by obtaining your own test kit. In 10 years of having my TFP pool I've never once had algae. I spend maybe a *max* of $150 a YEAR on pool chemicals and testing refills.

We need to know from a *good* test kit these elements-
FC
CC
pH
TA
CH
CYA
Salt level

We worry that pool stores sell you unnecessary chemicals which jack up the wrong things and mess water up..... so you end up buying more. We also never just "shock" our pools routinely like pool stores advise. We only add what our pools need, when they need it.

Please consider obtaining one of the two kits we endorse-
Taylor K-2006C and the salt test K-1766. from an online source. or
The TF-100ProSalt which uses Taylor reagents also- its available from. www.tftestkits.net

The TF-100 is a favorite kit because it has more of the reagents we use most often and less of the ones we use less.

If ever your pool needs a bump up of FC (free chlorine) please use liquid chlorine as it doesn't add any unwanted stabilizer or calcium.

Some fast reads-- ABCs of Pool Water Chemistry
FC/CYA Chart

Maddie 🐞
 

Donldson

TFP Expert
LifeTime Supporter
Jun 12, 2009
4,972
NW Ohio
Welcome to TFP!

I second the above, that sounds like dead algae accumulating. That happens when there's enough chlorine to kill algae at roughly the same speed it reproduces, creating an endless stream of sediment. Unfortunately that's somewhat common for those following pool store advice, as I found out myself way back when I found this site. Their recommendations just aren't based on good methodology and rarely are based on accurate testing.

As said above, the remedy will depend on what is actually going on with your water. We can help you determine that, we just need some trustworthy data to make that determination and get you on the right path.
 

atxdave

New member
Jun 10, 2016
3
Round Rock, TX
Pool Size
22000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
@JohnT, @YippeeSkippy, @Donldson - if I may ask - what is it about Nicola's description of the 'algae' that made all of you certain that it wasn't mustard algae? I ask because I'm about 5 days into a SLAM, and each time I visit the pool throughout the day, there are new collections of what I believe to be mustard algae. I would describe it pretty much the same way that Nicola did, though I've only ever seen it on level surfaces (steps & floor) and never on the walls. The amount re-appearing has decreased significantly since I started the SLAM, but it doesn't seem to want to go away completely.

Thanks!
 

JohnT

Admin
Mod Squad
TFP Expert
Apr 4, 2007
10,006
SW Indiana
Pool Size
21000
Surface
Vinyl
Chlorine
Liquid Chlorine
@JohnT, @YippeeSkippy, @Donldson - if I may ask - what is it about Nicola's description of the 'algae' that made all of you certain that it wasn't mustard algae? I ask because I'm about 5 days into a SLAM, and each time I visit the pool throughout the day, there are new collections of what I believe to be mustard algae. I would describe it pretty much the same way that Nicola did, though I've only ever seen it on level surfaces (steps & floor) and never on the walls. The amount re-appearing has decreased significantly since I started the SLAM, but it doesn't seem to want to go away completely.

Thanks!
Sediment on the bottom is a classic description of plain old algae that is just barely being held back or pollen. Mustard algae grows on the walls and bottom, primarily on the shaded side of the pool.
 

atxdave

New member
Jun 10, 2016
3
Round Rock, TX
Pool Size
22000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
Sediment on the bottom is a classic description of plain old algae that is just barely being held back or pollen. Mustard algae grows on the walls and bottom, primarily on the shaded side of the pool.
I see - thank you.