Dead mustard algae

by Dana
(Wheeling, WV, US)

My problem is that I can't get the dead gray looking algae out of the pool. I have a DE filter. I vacuum and backwash (many times now) and it keeps returning, usually in the same places.

Hi Dana. A full set of chemical numbers will be needed along with your pool size. If the mustard algae keeps coming back, it sounds like it was not properly treated.

If you vacuum to filter and particles are returning back to the pool through the returns, something could be wrong with the DE filter.

High doses of chlorine is the only thing that can kill mustard algae.

I also offer a variety of resources to help you with your immediate needs.

Clear Blue Pool eBook
How To Clear Up A Green Pool eBook
Personal Phone Consultation
Learn To Clear Up Your Pool


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Above Ground Pool Problem Yellow Water & Can't Fix

I went to Leslie's Pools because when I filled pool with well water it was yellow. We added metal free for iron and copper.

We waited 48 hours and they were gone and water was clear so we added two 3 inch tablets of chlorine and 16 oz. of dry acid.

They said my pH was 7.8 and my alkalinity was 110. I did this and after 24 hours water turned yellow. When I swept it you can see like a cloud of dust coming from the bottom of pool. You can wait a few hours and it is right back again and water looks yellow.

I went back to them and they said to use Yellow Out 8oz. wait 5 minutes and use 3 oz of powder plus shock. They said that should do the trick.

I watched it and I thought it was getting better before dark. However this morning it is yellow again and powder stuff is on bottom maybe even worse.

I swept it again and the water seems to even get yellower more so.


I never had this problem last year. I added metal free and chlorine and that was it. We do have well water. I have a summer escapes above ground pool with 1000 pump.

Thanks for the question

First I'll start with the readings. I have a pH level of 7.8 and TA of 110. These are good. I'll need to other readings as well - chlorine, stabilizer (CYA) hardness, and metals. The pool store should give you all of these readings and do it for free.

If the tests come back with high iron, you'll need to use a good metal sequestrant. Metal sequestrants that are based on HEDP, phosphonic acid and/or its derivatives are the most effective.

Some popular brands are Jack's Magic Blue, Purple, and Pink Pink Stuff, Metal Magic, Metal Free, & Metal Klear.

This is normally not a one shot deal. A metal sequestrant does not remove metal from pool water. It holds it in solution until it can get filtered. Then you backwash the metal out.

Because metal sequestrants break down over time and get filtered and backwashed out, you will need to add a bottle, or enough for your size pool, once per week.

Next, you need to determine is you in fact do have yellow algae. Allow the yellowness to settle then place a chlorine tab in the affected area for about 3 - 5 minutes. If the spot fades or gets lighter you have yellow algae and need to shock the pool. If the area doesn't get lighter or goes grey or black it's a high metal count. Use the metal products above for that and follow the directions on the bottle.

Shocking is a process, NOT an event. The trick is to get AND keep the chlorine above 10 - 12ppm for a period of time. You'll need to manually dose the pool with chlorine to keep it at 10 - 12ppm. Make the adjustment at night, then retest in the morning. Be sure to have the pump running 24/7 and backwash once per day. You can go up to 15ppm, but no higher. Anything after that and you're wasting money and chemicals.

You'll know this is working because the chlorine will begin to hold better. First you may lose most, then 2/3, then 1/2, and so on. Once you only lose 1 - 2ppm of chlorine 8 - 10 hours after the last application you know the algae is dead.

Now it's just a matter of filtering and backwashing once per day. Broadcast the chlorine around the perimeter of the pool and brush well. This will loosen up any algae adhering to the walls and bottom.

It's important to know the CYA because if it's too high, above 70 - 80ppm you need to do a partial drain and refill to get it to the proper range of 30 - 50ppm. By not doing this you're wasting alot of time and money because you'll be in the same situation in the near future. The CYA/chlorine relationship needs to be met.

If it's too low, below 20ppm, you'll need to use Dichlor to shock the pool. Once the 30ppm is reached, stop using Dichlor and go back to regular liquid chlorine.

To post a reply, or if you have a similar question, you can see your post on the Q&A page in the "Yellow Algae" category.

Check back to this post for updates or answers.

If you need immediate assistance (within 24 hrs) or for emergency personal assistance, you can make a donation of your choice and I'll answer your

questions by phone.

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Hope this helps and have a great Summer.


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