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Thread: Stain Keeps coming back in my fiberglass pool!

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    Stain Keeps coming back in my fiberglass pool!

    I have had a fiberglass pool with salt system and chlorinator for about 3 years now. The first year I had no problems whatsoever. Then I started getting stains on my pool which quickly disappear when I add ascorbic acid. My problem is that it happens about once a month and I don't know why. This has happened the past 2 summers. I've had my pool checked for metals and it shows very low amounts. In fact when I add the ascorbic acid I've tried putting metal free and I've tried not putting it in. The stain still comes back. I've tried to lower my PH and keep the chlorine level fairly low and the stain still comes back. No matter what I've tried, the stains still come back about every 3 - 4 weeks. It's kind of a pain because then I have to bring the chlorine level down to nothing, add the acid, filter for 24 hours and go about bringing the chlorine level back up again. My PH and alkalinity levels always remain pretty constant. Other than the stain, my pool is normally very easy to take care of. Is the stain returning that quickly something that's normal??? Am I doing something wrong?? Help!!
    Templehb - 16,000 gal fiberglass inground pool, Aquapure 1400 SWG, DE filter.

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    duraleigh's Avatar
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    Welcome to the forum.

    Since ascorbic clears the stain, you know it's iron.

    What levels are your pH, Alk, CYA and chlorine?
    Dave S. - Forum owner
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    JasonLion's Avatar
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    From your description you must be getting metal stains, so there must be metals in the water (except when the metal is in the stains instead of in the water). You need to use a sequestrant, both initial dose and routine maintenance doses, to keep the stains from reoccurring. I suggest starting with a top quality product like Jacks Magic Pink, Blue, or Purple Stuff or Proteam's Metal Magic and see if that works. If they do you can experiment with the less expensive products and see which of them work for you.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    PH is 7.6, Chlorine is 3.0, Alkalinity is 150 but I'm not sure about CYA.
    Templehb - 16,000 gal fiberglass inground pool, Aquapure 1400 SWG, DE filter.

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    Hey,

    a lower pH and lowering the Alk may help the stains from reappearing. I would defer to Jason and say you'll probably need the sequestrant anyway but you could first try keeping the pH around 7.0 - 7.2 and your Alk down around 80-90.

    Of course, you could drain and refill with metal-free water.

    Did you fill initially from a well? Are you still using the well as refill water?
    Dave S. - Forum owner
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    My care and maintenance book for my fiberglass pools says that my PH should not be that low. It says for a fiberglass pool it should be kept between 7.4 -7.6 and alkalinity between 125 - 150.

    No, we did not fill from a well. We are using city water. Can draining and refilling keep the stain from coming back?
    Templehb - 16,000 gal fiberglass inground pool, Aquapure 1400 SWG, DE filter.

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    Can draining and refilling keep the stain from coming back?
    If the fill water doesn't have metal in it, "Yes". Do you have any idea how it got there? That's a bit of a puzzle.

    I would be surprised if municipal water has that much iron but I suppose it certainly can....I just assumed it was always treated and the iron removed.
    Dave S. - Forum owner
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    JasonLion's Avatar
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    Keeping PH between 7.4 and 7.6 is ideal, down to 7.2 is fine, and while going down to 7.0 might cause some minor issues, like eye irritation, it isn't going to damage anything.

    There is no reason to keep your alkalinity that high. When using trichlor for chlorine TA between 100 and 120 is good. With bleach 70 to 90 is good. With a SWG you could go a little lower than with bleach.

    Metal staining is very very sensitive to PH. It is almost impossible to get stains around 7.0 and very likely at 7.8 to 8.0. Fiberglass is more prone to staining than vinyl or plaster, so the PH doesn't need to get that high to have problems. 7.6 is risking staining without sequestrant.

    Depending on the area iron is sometimes allowed in municipal water. Removing it gets expensive and if it is very common in the area they don't always bother removing it. You can ask your water company and they should be able to tell you.

    Copper is also possible if you have a heater with a copper heat exchange coil and would cause very similar symptoms.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    You did not list your calcium hardness. Almost all fiberglass manufacturers recommend between 200-400 ppm if you are going to follow your care book. However, the need for that has never really been established. There is some evidence that calcium levels below about 150 ppm can lead to cobalt leaching from the gelcoat (black pinpoint spots) and increased incidence of staining but no hard data.
    I used to have major iron staining problems similar to you with my fiberglass pool when I ran my calcium at around 150 ppm. As an experiment i have raised my calcium hardness to between 250-300 ppm and the iron stains have become MUCH less frequent and severe. In fact, I have not had to treat my pool with ascorbic acid for about a year now. I still add a metal sequesterant monthly.
    Your care manual might recommend a high TA BUT you have a SWG which changes things so you would benefit from lowering your TA to around 70-90 ppm.
    I run my TA at that range, my pH between 7.6-7.8 and also have borates to 50 ppm in my water.
    Hope this is helpful.

    Iron stains are often caused in a salt pool by using pool salt! Some of the very fine crystal pool salt has anti caking agents in them. I found this out the hard way by pouring a back of pool salt into my pool and was left with an iron stain where the salt sat to dissolve. This NEVER happened when I used solar salt and I have been fightng iron stains ever since. Needless to say I do not buy pool salt anymore! Iron can also be introduced from rusting of screws and other metal parts that are not stainless or very low grade stainless and not really proper for use in a salt pool.

  10. Back To Top    #10

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    Wow, you guys are awesome! I have so many new things to try! I have never tried dropping my TA and alkalinity so I'll try that first and see how it goes. I always assumed I should keep it as high as the book said to. I will keep you informed of my progress! Thanks again so much!!
    Templehb - 16,000 gal fiberglass inground pool, Aquapure 1400 SWG, DE filter.

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