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Thread: Can not maintain a fc level

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    Can not maintain a fc level

    I have a 27ft above ground pool with a vinyl liner. I am using a system called Sustain which is a calcium hyplchlorite based which i am having to supplement bleach to maintain a fc level. When i opened my pool about 2 weeks ago, i noticed some mustard algae in the bottom. So I purchased a yello free product that is activated by shock from my local store. I did have some mustard algae last summer that I had to treat. So i dosed my pool with 13lbs of shock over a period of couple days and by end day my FC would be 0. so i found your site and purchase 13 gals of bleach. Over last wkend I added the bleach trying to maintain a high shock level. Still no luck holding chlorine. I have tried the overnight test with just test strips. Last night to the best of my knowlege i did loss a small amount of chlorine. But it also rained here during the night. Had my Water tested today at pool store around 11;30a.m. Here are the results:
    FC 2.9 - but had justed added about 1/2 gal bleach 1 hour prior
    TA 94
    PH 7.4
    CH 190
    CYA 36
    Water is crystal clear and has been since i treated it for mustard algea and shocked it
    Went out to test water with strip at 2:00pm and the FC was 0
    I seem to lose the most during the day with sunlight.
    Im at a loss with what is going on. My pool store has no idea. We did test for phosphates but don't think there was enough in water worth treating. Also, Ive purchased the strips to test for ammonia. The results read maybe .25ppm. This was before I added my 13gallons of bleach. Retested, hard to tell but think it is 0ppm. Please help.
    20,000 gals, 27ft above ground, vinyl liner, 1.5 hayward pump and sand filter

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    Re: Can not maintain a fc level

    Hello and welcome to TFP!
    FC will be used up by either algae or sunlight. Where do you live? It your pool in direct sunlight? CYA will help keep chlorine in your pool..and 36 is in the proper range...BUT I am always skeptical of pool store results.

    I see that you have not tested for combined chlorine..those numbers will help indicate where the FC is going.

    It would also be helpful if you add pool and equipment specs to your signature box. It might be worth looking into getting your own test kit. The TF-100 and Taylor K-2006 both come highly recommended on this website.

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    Re: Can not maintain a fc level

    Thanks for responding! My pool is in direct sunlight! And I live in the upstate of South Carolina. I am looking a purchasing a test kit. I have tested for cc but the result would not be accurate bc with the sustain system you add a liquid sun shield. And the Sun shield effects the cc reading. Hard to explain. But with this system I should have a cc total of at least 1.5ppm. My CYA reading was 54 last week according to the pool store. But we have had a lot of rain and Ive had to drain some water bc of all the rain. Plus ive backwashed a few times.
    20,000 gals, 27ft above ground, vinyl liner, 1.5 hayward pump and sand filter

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    Re: Can not maintain a fc level

    Ok, well it sounds like the sunlight is a factor for your FC loss.

    As for the CYA...am I understanding this correctly:
    Pool store said 54 last week
    Lots of rain since then
    Now pool store measures CYA at 36

    If that is the case, then it seems like either A) you got a LOT of rain and a large portion of pool water was displaced, or B) the pool store readings were/are not very accurate.

    Since we already know that B is true...I come full circle to recommending a good test kit. Until then, I would continue to monitor your FC - making adjustments based on the Pool Calculator.

  5. Back To Top    #5

    Re: Can not maintain a fc level

    Ok- so i received my T100 kit this afternoon. I tested the water around 5:30pm. Here are the results:

    FC- 1
    CC - 1
    CYA- 34- maybe (closer to 30 than 40)

    So I wanted to shock my pool at mustard algae level bc Iv had a problem with that. I was aiming for around 21ppm FC. I used the calculator and it suggested 830 oz. So I added 5 (182 oz) jugs of 6% walmart bleach. I waited 2+ hours and rechecked:
    To my surprise my FC is around 33 and CC .5. So now Im concerned that my FC is too high. On the positive: I think Im holding my chlorine longer. Just need to get rid of what I think is some mustard algae on the bottom of my pool.

    So my question: Why did my FC rise so high and Is this a concern ?
    20,000 gals, 27ft above ground, vinyl liner, 1.5 hayward pump and sand filter

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    JasonLion's Avatar
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    Re: Can not maintain a fc level

    There are several things that could have thrown off the calculations. Most likely, your pool doesn't hold quite as much water as you think it does. For 27' round, 4' deep I get 17,000 gallons. The bleach could be stronger than it is labeled. Bleach starts stronger when it is brand new, and then the level goes down slowly over time. There is also a small chance you are doing the FC test wrong.
    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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    Re: Can not maintain a fc level

    My pool is sloped deeper to about 5 ft. in the center and has 52" side walls. So, it may not be quit 20000 but should be close.

    Is there a need to be concerned about high level. Just did my OTO test and chlorine is a dark orange.
    20,000 gals, 27ft above ground, vinyl liner, 1.5 hayward pump and sand filter

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    Re: Can not maintain a fc level

    Ok- so Ive been monitoring my FC level for 2+ days. The following is my log:

    On Friday around 6 pm- Received my test kit
    FC: 1
    Cc:1
    CYA:34
    Added 5 (182oz) jugs of bleach

    Fri. 9:00pm
    FC: 33

    Sat - 8:00am
    FC- 32.5
    Sat - 1:10pm
    FC: 25
    Sat - 4;15 pm
    FC: 16.5
    Vacuumed and backwashed added 3/4 of 182oz jug of bleach
    Sat - 5:15 pm
    FC: 19.5
    Sat - 11:30 pm
    Fc:14.5

    Sun - 7:30 am
    FC: 14
    Sun - 12:40 pm
    Fc:8
    Sun - 3:40pm
    FC: 2.5
    CC: 1

    So my question is: Should I be losing this much chlorine in one day. I went from 14 this morning to 2.5 a few hours ago? My pool is in the sun and my CYA is around34.
    20,000 gals, 27ft above ground, vinyl liner, 1.5 hayward pump and sand filter

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    Re: Can not maintain a fc level

    Chlorine is consumed by ONLY two things...

    1. The sun With a CYA of 30 or so, you will probably lose 3-5ppm daily

    2. Organics in your pool. A loss greater than what the sun consumes can only be attributable to algae or some other organics that are in your pool. You need to continue the shock process.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
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    JasonLion's Avatar
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    Re: Can not maintain a fc level

    It varies quite a bit from pool to pool, but roughly speaking you are going to lose about 2/3rds of your chlorine over the course of a sunny day with CYA around 30. So your chlorine losses are on the high end of plausible. To cut your daily chlorine loss you can raise your CYA level. We recommend a CYA level of around 50 to start the season (it will drift down slowly from there as water is lost).

    What is interesting is how much chlorine you lose overnight. From what I see in your chart you are losing about 0.5 overnight, which is good.

    I don't see anything out of the ordinary going on here, nor any sign of algae.
    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

  11. Back To Top    #11

    Re: Can not maintain a fc level

    Thanks for the replies! Yes, I am maintaining a low chlorine loss over night. Which from what Ive read is a good sign that it is not algae but the sun.
    So I've got two different responses. I had planned to raise my CYA to around 50 once I completed the shock process. But I don't want to raise CYA if I have to continue the shock process bc then it will require higher levels of chlorine to shock.
    Do I need to keep shocking or raise my CYA? If I need to keep shocking then, for how long? My water looks great!
    20,000 gals, 27ft above ground, vinyl liner, 1.5 hayward pump and sand filter

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    JasonLion's Avatar
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    Re: Can not maintain a fc level

    You are done shocking when you pass the overnight chlorine loss test, which you have, have CC at 0.5 or lower, which may be a problem, and don't have any visible algae, which I gather you don't.

    If I'm reading your chart correctly, CC has been at 1 for a while.
    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

  13. Back To Top    #13

    Re: Can not maintain a fc level

    Yes, cc is at 1. But that is not an accurate judgment. Because I started out using the Sustain Pool Chemicals. The first thing you do is add a summer shield to the water. This affects your cc reading. Because your summer shield reading is = to your CC.
    20,000 gals, 27ft above ground, vinyl liner, 1.5 hayward pump and sand filter

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    JasonLion's Avatar
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    Re: Can not maintain a fc level

    Then you should be done shocking.
    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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    mcoonan's Avatar
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    Re: Can not maintain a fc level

    Were there any swimmers in the pool over the time you were measuring the fc?
    Intex Ultra 18' x 52" AG 6,981 Gal.
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    Mod Squad Bama Rambler's Avatar
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    Re: Can not maintain a fc level

    I looked at the sustain pool care system and I don't know what's in there that could cause a cc reading. All of it but the summer shield is just cal-hypo. Summer shield is 85% salt brine and 15% "proprietary"
    Dave J. TFP Moderator
    24' x 52" Round AGP. 2hp/¼hp SPL Power-Flo 2-speed pump. 200sqft Waterway Cartridge Filter. 45MHP2(3GPD) Stenner Peristaltic Pump
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    Re: Can not maintain a fc level

    Is there a cover on the pool?
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    You're done SLAMing when:
    1)You lose 1ppm or less FC overnight, & 2)You have .5ppm CC's or less, & 3)your water is clear.

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  18. Back To Top    #18

    Re: Can not maintain a fc level

    So answers to your questions. No there were no swimmers in the pool and no cover over it.
    20,000 gals, 27ft above ground, vinyl liner, 1.5 hayward pump and sand filter

  19. Back To Top    #19

    Mustard Algae

    Hi! Ive got a few questions concerning mustard algae. I had this problem last year and Im wondering if its back. I have vacuumed my pool twice today. We just got back from vacation and I had a lot of dirt/ red mud dust in bottom of pool. I vacuumed and a couple hours later it was back. However not has much as previous. I vacuumed a 2nd time and with a few hours I noticed more dirt. This time even less than before and not red looking. It settles on the bottom of pool, all over in dipples and creases of liner. Have not noticed any on pool walls. It is not slimy and maybe poofs a little, not much. I shocked my pool and FC is 14 and debris is still on bottom. So questions are : If this is MA will my FC level drop overnight? And Ive read that when FC is maintained at high levels that MA will not appear. Is FC of 14 considered high enough?
    FC - 14
    cc - .5
    CYA- around 42
    CH- 590
    TA - 90
    Ph- 7.2 just lowered it today

    Or can I assume this is just dirt / debris?
    20,000 gals, 27ft above ground, vinyl liner, 1.5 hayward pump and sand filter

  20. Back To Top    #20
    linen's Avatar
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    Re: Can not maintain a fc level

    It sounds like dirt/debris to me. Mustard algae usually also grows on the sides mostly in the shady portions of the pool. If you can get a sample, see if it feels slimy...if it does, it is probably algae, though I would guess not MA.
    TFP Expert who uses Pool School and my TF100 test kit along with PoolMath for my: Round 11K gallon AGP with deep end, 20" sand filter, Matrix 1hp 2spd, 6 2ftX20ft solar panels (and solar cover!), Intex SWCG (copper bars disconnected) and a Rubadub hot tub (chlorine). The SLAM process is not finished until: 1. CC < 0.5 ppm, 2. An OCLT < 1.0 ppm and, 3. The water is crystal clear.

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