Grey Debris In Pool

by Nancy Rush
(Norwalk, CT, USA)

pool debris collected from vacuum

pool debris collected from vacuum

Let me start by describing my pool. It's an Endless Pool, i.e. it has a propellor in the front that generates a current when on. This allows you to swim in place for exercise. The pool is only 7'x14' and about 4.5' deep. It's got a vinyl liner and benches around the perimeter in which the water from the current returns to the propeller unit. I keep the pool at 76 degrees F and there's about 2700 gallons of water in it. It's an indoor pool with no CYA. I keep it covered whenever it's not in use.

Now for all of the details! For the last 2 months I've been struggling with the water. There is some kind of soft, grey debris in the water. It appears as white flakes or strips of cloth or paper when floating throughout the pool; but, captured in my vacuum net, the debris is clearly grey and soft, and almost dissolves when rubbed between the fingers.

After much research, I assumed this was white tissue mold and so I added enough Clorox bleach to raise the FC to over 10ppm. The FC remained above 5ppm for a full week. I had only a DPD test kit so could not get an accurate reading. Once my new Taylor 2006C test kit arrived, I confirmed that the FC was at 6.8 ppm and CC at 0.2, PH at 7.8, TA=105 and CH=210. Over the next 2 days, the FC remained above 6 ppm, but the TA reduced to 90. Then, since I had to leave town for a couple weeks, I added more Clorox bleach bringing the FC to over 12 ppm. I also replaced the filters (2 Pleatco PWW10-M) and added baking soda to bring the TA up to 110 and confirmed the PH was at 7.2 just before I left.

Two weeks later, when I returned home, I uncovered the pool and retested. The FC was 8.0, CC=0.4, PH over 8, TA=110, CH=200. I vacuumed and brushed the pool, added some acid to bring the PH down to 7.4 and tried to capture as much debris as I could using a net with the propeller running. Since I had read a lot about non-chlorine shock, I decided to try it since the high levels of FC didn't seem to be resolving the problem.

For some reason I thought it would be best to have lower levels of chlorine in the pool when I used the MPS shock. So I added dechlorinizer to the pool to bring the FC down to 4.8. Then I added MPS (Fresh 'N' Clear), let the propellor run, and retested about an hour later. The FC was now reading at 7.2 and the next day the water became slightly cloudy. So I replaced the filters, vacuumed, brushed and retested: FC=7.0, CC=0.0, TA=100, CH=200.

At this point I tested and brushed the pool daily for another week while the FC slowly reduced, adding acid as needed to keep the PH at 7.4 (or as near as possible). After a full week, the FC was at 5.0 and the TA had reduced to 90. I replaced the filters again, added baking soda to raise the TA to 100 and continued daily testing and brushing for 4 more days until the FC was at 3.6.

During this time it appeared as though the debris was gone, or at least had significantly reduced. I decided to shock with the MPS again to be sure to the pool was clear of any organic material. After adding the MPS, lots of debris started to appear in the pool again and the FC was measuring at 5.0 (up from 3.6 prior to the MPS).

The next day I replaced the filters and retested (with my FAS-DPD test). The FC was still at 5.0, CC=0.2, PH=7.6. I added more MPS and afterwards the FC went up further to 6.8. Two days later the FC was still up at 6.2, CC=0.2, PH=7.6, and CH=190. I added a small amount of acid.

The next day the levels had barely changed: FC=6.0, CC=0.0, PH=7.5, TA=100. But there was lots of debris in the pool that I continued to vacuum daily. I decided to add more MPS and afterwards the FC tested at 7.4! I didn't understand why the FC increased whenever I added a chlorine-free MPS shock.

Finally online I learned that MPS can interfere with the FC FAS-DPD test. Presumably, it should only interfere with CC using a DPD test, but my Taylor 1005 test kit was also reading >5 ppm of FC and no CC. Since the debris was so bad and I hadn't added any Clorox to the pool in a full month, I worried that perhaps there was no FC in my pool despite the test kit readings.

So to be sure, I added 2 C of Clorox to the pool and retested after letting the propeller run for a while. The FC was now measuring at 11 ppm with CC=0.0.

For the past week, my regular routine has become, 1) open the pool, all the windows, turn on the ceiling fan, turn on the HRV, and turn on the propellor to get the water moving, 2) after 30 min the propellor would shut off, I would test the water, brush the walls, and vacuum whatever debris I
could capture, 3) turn the propellor back on and add acid if necessary to keep PH in range, and 4) close the pool and all windows after about total of 2 hours.

The debris remained and didn't seem to be improving. The chemicals are currently measuring at FC=9.5, CC=0.0, PH=7.6, TA=85 and the water temp is 76. There is still a lot of debris in the pool, seemingly larger pieces than before.

Again worried that the FC measurement is still unreliable, I just added another cup of Clorox (6% chlorine) to the pool. And I'll add baking soda shortly to bring the TA up. I'm at my wits end. I've been collecting the debris captured by my vacuum net in a glass of water in my kitchen. It's a lot of soft grey flakes, with occasional strips, sometime half an inch long.

Why is the MPS causing my FC reading to increase and remain high even days later? Why does the problem appear to be worsening rather than improving? Is this white water mold or something else?

The water has been crystal clear throughout (with the exception of a day or two following my first MPS shock). The vinyl walls are literally squeaky clean and I can't see any kind of filmy substance on the water surface or on any of the pool surfaces (I've read that white mold often is accompanied by a bio film).

The only other thing to note is that at the water line within the skimmer, there is a white/greenish substance that collects daily. I've been wiping this off with a paper towel and throwing the towel away. It's not appearing at the water line anywhere else in the pool.

During the last 3 filter replacements, the used filters have not appeared very dirty. There is some white substance between some of the filter folds, but not much. I've replaced the filters weekly, throwing away the old ones, just to be safe.

I'll attach a picture of the debris I've collected. I would be so grateful for any advice or if you have any idea what this might be! The debris that I collected in a glass has not grown like I would expect mold to.

Oh, and one final detail: when I discovered last week that MPS can interfere with FC readings from an FAS-DPD test, I ordered a DEOX reagent to eliminate this interference; however, it won't be delivered for at least another week.

Thanks for the question Nancy

All of the details you describe in the question can point to white water mold. This usually happens with hot tubs due to the increased water temperature but can find its way into pools.

Water Mold

Concerning the MPS, this is monopersulfate which is an oxidizer and can work in tandem with chlorine, or bleach which is what you're using.

MPS will not show up on the FAS-DPD test as chlorine, it shows as combined chlorine, or CC. When the CC goes to zero the MPS is gone. Because MPS is acidic it will decrease you pH and TA so be careful when using it.

Your chlorine levels are relatively high, but not high enough to kill water mold. 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 remove the filters every other day. I assume you have a cartridge filter. You can go up to 15ppm, but no higher. Anything after that and you're wasting money and chemicals.

For bleach, a good rule of thumb is 1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm. I'd raise it to 1/4 teaspoon. For liquid chlorine, it's 10 oz. per 1,000 gallons for 10ppm. Your pool is 2,700 gallons so you'd use 30 oz. of chlorine. It's better to go just a bit higher than lower.

First you may lose most of the chlorine because it's eating up the water mold. The next day or two later you might only lose 2/3 chlorine, then 1/2, and so on. Once you only lose 1 - 2ppm of chlorine 8 - 10 hours after the last application you know the mold is dead. Now it's just a matter of filtering and changing out the filter. Broadcast the chlorine around the perimeter of the pool and brush well.

If you want an acceleration of oxidation due to the water mold issue, you'll need to maintain a somewhat higher chlorine/CYA ratio to avoid the use of MPS or additional products.

The CYA is 7.5% of your chlorine and should be 30 - 50ppm. If you run the chlorine at 2.5ppm, simply divide 2.5 by 7.5% and you get 33.

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

Check back to this post for updates or answers.

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Hope this helps.


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Jan 29, 2013
No CYA, indoor pool
by: Nancy Rush

Hi - Thanks for your reply. I have a couple follow-up questions. If MPS should not influence my FAS-DPD test for FC, then what is making the FC measure at higher and higher levels after each addition of MPS to the pool?

From the start, and throughout, the FC level does not reduce at a very quick rate. Since mine is an indoor pool, there is no CYA in it and it gets no exposure to the sun to burn off chlorine. Recently when I added chlorine to the pool to raise FC to 11.5, it remained at this level the next morning. Also, by the next day it only reduced to 10.5. Does this indicate that the mold is dead since the chlorine demand is not that high?

Thank you for your help!

Aug 28, 2014
Not mold after all - Degrading Hydraulic Hoses in Endless Pool
by: Nancy Rush

I just wanted to follow-up on this thread. While the water chemistry information in this thread is generally useful, and I'm grateful for the info and advice, I just wanted to let other readers know that I finally discovered what was behind the debris in my pool.

It turns out that the submerged hydraulic hoses in my Endless Pool had been slowly disintegrating. The outer coating of the hoses turned from black to a light grey and started to flake off and float into the pool. This wasn't a mold at all, but instead a faulty product.

So beware existing and future Endless Pool owners! If you have what you think might be white water mold, or tissue mold, examine those hydraulic hoses because they may be disintegrating!

Thanks for the follow up Nancy. I would have never thought of going that route nor have I ever heard of such a thing. That's why we're never too old to learn something.

Great advice.


Aug 24, 2015
Endless Pool Owners beware
by: Dan

We encountered the same disintegrating pool hose situation and finally found the cause for the months of debris thanks to the people at Endless Pools!
I wanted to ask Nancy what did she do next and did Endless Pools know about the problem and what, if anything, they did to help? Thanks!

Oct 04, 2015
Help! We have the same problem
by: sydney long

WE have the same problem and would love to know how Nancy ( or anyone) resolved it!


Oct 29, 2015
Endless Pool hydraulic hose resolution
by: Nancy Rush

Hi Sydney - In my case, I had to insist that Endless Pools replace my hoses. They initially tried to put me off. I had to escalate the issue to the one of the heads of customer service. I called the number published on the BBB site for Endless Pools which connected me to a very professional individual in Customer Relations. She responded with appropriate dismay to my circumstance and how poorly it was handled initially by customer service. She then helped me escalate the issue to the appropriate person.

I was told that the entire customer service department would be made aware of my case so that they would be better able to service any other clients that experienced the same problem going forward. I was also told that they would research the case with the hose manufacturer and let me know what they found. I have not heard from them since; however, the replacement hoses are holding up much better.

Good luck to you - I hope you are able to get EP to take responsibility for any faulty hoses in your pool!

Oct 29, 2015
by: Sydney

Hi! So good to hear from you. This is very helpful.

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