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Blue slime


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While trying to balance the chemicals in our pool after it was shut down due an equipment failure the water turned cloudy and light blue.  We ran the filter overnight and the filters were clogged with a cyan blue slime.  The pool was clear before the incident.  Ph was low, below 6.8.  Free chlorine was not registering but total chlorine was high.  I added Ph up using the recommended dosing.  Later the same morning, we added a small dose of shock and that seemed to initiate the problem.  Any ideas what happened?





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pH was below 6.8 but you don't know just HOW low. My guess is that your water was VERY acidic

You don't give any test results so I really can't comment but I have seen this before when the pH was dangerously low. I noticed the endcaps of your filter are blue. Are they by any chance gummy feeling or slimy feeling? I have seen very low pH 'dissolve' filter endcaps and other parts and I suspect that is what happened.  It often occurs when someone puts trichlor tabs, which are extremely acidic, in the skimmer basket, or even more dangerous, into the pump strainer basket. However, without knowing the actual water parameter when it happened it's impossible to really know what happened in your pool but it sure seems like very low pH caused something blue to dissolve.


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  • 4 months later...
On 8/27/2021 at 8:38 AM, Jeffrey W. said:

I have experienced the exact same problem and can confirm that my pool pH was NOT dangerously low.  I check daily and it is generally around 6.8 or slightly higher. I wish I could figure out what is going on!


Different color of blue. Do you use a copper based algaecide or have an ionizer system by any chance? Also how are you testing pH?

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  • 8 months later...

This is 100% the filter end caps with the soft grip “dissolving” from imbalance. Watch out because it doesn’t redesolve with filter cleaner and has to be manually sprayed or scraped off. Also, watch out for clogged lines after this and blocked sensors on pumps and thermometers. And honestly, it’s probably due to a change in material by a Chinese manufacturer. I’ve seen it many times before. 

Edited by EricHei
Wanted to add root problem.
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  • 2 months later...

My pool filter looks exactly the same as DWest filter. I have a mustard algae problem that's almost under control. I used Clorox Xtra blue algaecide,  which is a liquid. I'm assuming that's what clogged my filter? But why did it do this? It's a very thick heavy slime completely coating the filter. I only put the algaecide in once but 2 times now my filter has clogged up like this. Water balance is perfect.

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On 7/8/2022 at 6:54 PM, Dmh said:

Water balance is perfect.

this tell us nothing. We need a full set of test results along with the testing method (strips, liquid or tablet reagents, test disc. Whether the results are read from a color block or with a meter. Is this a home test or dealer test.) pH issues can cause the problem you describe as can.

Without further information on your water my best guess is that you shocked the pool very high because you were trying to kill mustard algae and then tested pH without allowing the chlorine to drop below 10 ppm first, which is a no no because of the effect of high sanitizer levels on the pH test.

Hgh sanitizer causes the phenol red indicator to convert to chlorophenol red which has the same color changes as phenol red but test a pH range of  4.8 to 6.7 so when the pH indicates that it's 8.0 or higher all you really know is that it's greater than or equal to 6.7 and when you add the acid you are bringing the pH below 6.7 which is low enough to dissolve the rubber end caps on the filter.

I suspect you then put in acid because the pH read very high after the shock, even though it was not and brought you pH dangerously low, which caused the end caps on the filter to dissolve and get caught in the filter.


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  • 1 year later...

We have a very similar problem except our slime is black not blue. This has happened twice in 8 months and the only solution was acid washing the pool. Does anyone have any idea what is causing this?  Our pool service indicates all chemical levels are normal and they have no idea what the cause is.

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@Sarah Fix you posted in a dead year old thread. Please start a new thread so you have a better chance of getting an answer. When you start your thread please post how you are sanitizing (chlorine tabs, salt water chlorine generator, liquid chlorine/bleach, diclor, cal hypo) and a full set of test results including cyanuric acid (stabilizer)m free chlorine, combined chlorine OR total chlorine , pH, total alkalinity and calcium hardness and if you use a non chlorine shock. Also post how the test results were obtained (test kit with liquid reagents, test strips, strips or reagents  with a meter or strip reader, or dealer testing (please specify if they are using liquid reagents, a test disc in a meter, or strips (with or without a strip reader).

I have some ideas about what might be going on but need to know your water parameters first to narrow it down. I will answer in the new thread you start once you post the information.

Also, posting a picture of the 'slime' would be very helpful in identification.

That being said:

5 hours ago, Sarah Fix said:

This has happened twice in 8 months and the only solution was acid washing the pool.

Which involved draining all the water, refilling and rebalancing the chemicals, which would temproarily correct any water balance problems such as overstabilization. I am going to guess that you are either using trichlor tabs in a feeder or floater or using dichlor for either chlorination or shocking. I am also going to guess that you have a cartridge filter or a non backwash DE filter. You are in Florida in St John's county so you do not close your pool. I am also in St Johns county so I am very aware of the special issues Florida pool owners face.

5 hours ago, Sarah Fix said:

Our pool service indicates all chemical levels are normal

What is "normal"? What is their acceptable levels for cyanuric acid and free chlorine? A sad fact is that many pool service techs do not really understand the chemistry involved in pool maintenance and there are many pieces of misinformation that refuse to die in the the industry.

5 hours ago, Sarah Fix said:

they have no idea what the cause is.

No surprise here. As I said many pool techs don't really understand pool water chemistry.




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