Yellow/Mustard Algae-Why is this happening to me?

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
With what appears to be otherwise very good test numbers, it's hard to know why algae returns, especially MA. Various dynamics could come into play such as localized organic debris, unusual stress on the chlorine at some point, inadequate circulation, or perhaps algae that was undetected and simply resurfaced. Once a regular SLAM Process is complete, we shouldn't see any visible algae, so all that "might" be left is residual MA spores that require the higher FC level for that 24 hours. But either before you go to MA level, or after the FC falls back down under the regular SLAM FC level, inspect every inch of the pool one more time. Don't forget to remove the drain cover if you have one and check in there. Also behind a light (niche if installed) and hollow ladder or steps. Good luck.
 

YippeeSkippy

Mod Squad
LifeTime Supporter
Jan 17, 2012
14,580
Evans, Georgia
mustard algae tends to appear on shady areas of the pool- is that how you find it on yours? Perhaps leaving the FC slightly higher than normal after the SLAM process and making sure to brush well will help avoid it reappearing.

Maddie
 

Mypool12

Active member
May 20, 2017
38
Central New Jersey
With what appears to be otherwise very good test numbers, it's hard to know why algae returns, especially MA. Various dynamics could come into play such as localized organic debris, unusual stress on the chlorine at some point, inadequate circulation, or perhaps algae that was undetected and simply resurfaced. Once a regular SLAM Process is complete, we shouldn't see any visible algae, so all that "might" be left is residual MA spores that require the higher FC level for that 24 hours. But either before you go to MA level, or after the FC falls back down under the regular SLAM FC level, inspect every inch of the pool one more time. Don't forget to remove the drain cover if you have one and check in there. Also behind a light (niche if installed) and hollow ladder or steps. Good luck.
Thank you for your reply. This morning my FC overnight test showed a chlorine loss of less than 1 ppm. The loss was just .5. I am assuming this reading was because it was yellow mustard algae and my chlorine was in good shape before I started the SLAM. Should I now move onto the next step and raise my chlorine shock level to the yellow mustard level for the next 24 hours?

Thank you again for your help.
 

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Should I now move onto the next step and raise my chlorine shock level to the yellow mustard level for the next 24 hours?
It's tempting, and you could if you wanted since your FC is probably still elevated from the SLAM. It's just a little bit more anyways. But remember that you just started the SLAM Process yesterday. Even though you passed the OCLT, one of the criteria is no visible algae. So technically you didn't complete the regular SLAM. It would be ideal to remove all "visible" algae before doing the MA increase. Good luck.
 
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red-beard

Gold Supporter
May 27, 2019
553
Houston, TX
On the "maintain", I had Mustard Algae issues for years. This year, I finally have my CYA under control. I kept it on the lower end on purpose at 30 CYA. I then left kept my target 8 ppm FC. Each evening (after sun is off the pool) I would measure and then raise to 8 FC with my addition. I did this for 6 weeks and I did not have a M.A. outbreak. I lowered my target to 7 FC and then 6 FC over the rest of the summer. No M.A. outbreak. I think a high SLAM then keeping the FC higher for a while helps. I put together charts from the CHEM GEEK charts which show that at 30 CYA, you need to keep the FC above 4.3 ppm. I will check when I get home, as the chart is on my laptop.

Here is the previous discussion of a HOCl of 0.07 ppm

 

red-beard

Gold Supporter
May 27, 2019
553
Houston, TX
I'm home. Here is CHEM GEEK's chart, with the 0.07 HOCl horizontal line added for Mustard Algae. I also added some vertical lines to show the FC required for different CYA levels.

FCL & sanitization.jpg