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Mustard Algae: What is it? How do i kill it?

Updated: May 6



Removing Mustard algae in swimming pools
Mustard Algae

Few things are as aggravating to a pool owner as mustard algae. Mustard algae is an algae that grows an immunity to regular chlorine and shock. It is a brownish, yellowish, poofy looking algae that normally grows on the bottom and sides of the pool. When you brush it, it poofs up and disappears - only to reappear the next day. This algae usually appears in the hottest months of pool season - usually July and August.

Mustard algae cannot be killed with regular chlorine and shock. It needs a special 1 - 2 punch of a certain type of chlorine - dichlor, along with a specialty algaecide such as Algatec.

Dichlor is a form of chlorine that mustard algae is not used to. Hitting it with this form of chlorine is like hitting it with an antibiotic - effectively killing it with the algaecide.

When treating for mustard algae, you must adjust your pH between 7.2 and 7.6 for optimum results. Vacuum the visible algae out to waste before you begin treating for it.


This is a 3 treatment process:

1) First Treatment: use 2lbs of dichlor and 1/2 gallon Algatec per 15k gallons of water. Brush the pool thoroughly and put ALL of your pool equipment (nets, brushes, poles, floats, and vacuum heads or robot vacs) into the pool to be treated as well.


2) 2nd Treatment 12 hrs later, add 2 more lbs of dichlor per 15k gallons and brush again


3) 3rd Treatment 12 hrs later, add your last 2 lbs of dichlor per 15k gallons and do a final brushing


*Always wash bathing suits after swimming in lakes, rivers, or other pools before using them in your pool. NEVER CROSS - CONTAMINATE! Keep your lake floats separate from your pool floats.

Once you treat for mustard algae, you won't want to do it again. It's expensive and a pain in the rear. Take precautions to avoid cross contamination and keep your pool well chlorinated and balanced through the season.


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