Powdery Sediment On Bottom Of Swimming Pool..Yellow Algae..

by Brad
(Riverdale, GA)

After having my water analyzed I added soda ash, then alkalinity up and finally Phosfree. I ran the pump for 48 hours and the water turned cloudy.

We turned off the pump and after a day, the pool cleared with a powdery yellow sediment on the bottom.

Is this yellow algae? I have never used Phosfree before. The pool clerk could not explain/ understand what happened. Can you?


Thanks for the question Brad

First I would say that using different chemicals to raise your pool pH level and total alkalinity might be a waste of money. One of the best chemicals (if you can call it that) would be sodium bicarbonate, or normal baking soda. You can use this to raise both your pH level and alkalinity.

Many people have had very good success with 20 Mule Team Borax to raise the pH level and that doesn't affect the total alkalinity very much.

Swimming Pool pH Levels

Swimming Pool Alkalinity

Next is to get a Taylor Reagent FAS-DPD K-2006 pool water test kit. This is the kit most pool professionals use and it's the most accurate.

Water Testing Kit

As for Phosfree, I'm familar with this product, and here's the truth about it:

Swimming pool algae needs nitrates and phosphates to grow, reproduce, and spread, because this is the main source for pool algae.

Now, if you keep your pool chlorine level (the pool sanitizer) between 1.5ppm - 3.5ppm and cyanuric acid between 30ppm - 50ppm (the stabilizer for the chlorine), there won't be any algae or green pool water because the chlorine kills the algae.

If the pool chlorine kills the algae, what difference does it make what the phosphate level is? There's no algae to eat up the phosphates.

My YMCA pool has a pretty high phosphate level, I think it's over 1000, but it doesn't concern me because I keep the chlorine level up between 3.5ppm - 4.5ppm. This way there's enough chlorine to kill any organic matter and bacteria.

The yellow substance could be residue from the alkalinity up and soda ash. Normally yellow algae (sometimes called mustard algae) can be found in the shady parts of your pool and is swept away easily.

A good way of finding this out with your pool is to give it a good sweep, allow the dirt and debris to fall to the bottom, then vacuum the pool. You can either vacuum through the FILTER cycle or to WASTE. If you vacuum to FILTER be sure to backwash.

A day or two later come back and take a look at your pool. If you find yellow spots again and they can be easily swept away, you probably have yellow algae. If there's no yellow, then it might have been a reaction to the 3 chemicals you put in the pool, but to the best of my knowledge there's really no way of knowing, if you can't duplicate it.

If it is yellow or mustard algae you can go here to find the best way to kill it:

Yellow Algae

With respect to pool clerks, I've found that the best place to get swimming pool care advice is from someone who has taken care of pools and knows the "ins-& outs" of pool water maintenance from real experience.

Keep you pump running a minimum of 10 - 12 hours per day in the Summer, or you could invest in a variable speed pool pump that can stay on 24/7 but uses less electricity because it's not going "all-out" all the time.

Best of luck with your pool.


Question: White Sediment
Date: Oct 10, 2011

I have a white sediment at the bottom of my fiberglass pool. The water has been tested for everything and is perfectly fine. It is driving me crazy because I cannot figure out what this white stuff is... no matter what I do, I cannot get rid of it. Can anyone help me out!!

Thanks for the question

It would be helpful if we had your chemical readings, Chlorine, CYA (cyanuric acid/stabilizer), pH, Alkalinity, Calcium Hardness, and Metals (iron and copper).

It makes troubleshooting much easier and the process of clearing up your pool will go much faster. Also the kind of filtration system you have.

Comment: Just A Possibility
Date: Apr 12, 2012

I just happened to see this feed while trying to figure out a water softener issue but I'm wondering if the white powdery substance might be dead algae.

This has happened in my fish tank after treating it for phosphates and after adding algae fix.

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White Powder On Sides Of Pool

Last year we drained the pool then it was power washed and repainted.

This year there had been a huge amount of pollen in the pool, I think we have mustard algae because when you touch or brush it the pool is cloudy.
When you run your fingers along the pool in some places there is white powder residue.

First, I'd like to have your complete chemical readings, the actual numbers:

Chlorine, CYA (cyanuric acid/stabilizer), pH, Alkalinity, Calcium Hardness, and Metals (iron and copper) and names of any algaecides you may have used along with clarifiers, phosphate removers, and/or flocs. It makes troubleshooting much easier and the process of clearing up your pool will go much faster. Possible yellow algae is a symptom of the problem, not the problem itself.

You can get this done at your local pool store. Without these numbers I'm just guessing at what the problem is. Get back to me with the chemical numbers and I'm sure I can help.

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