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Thread: Can't keep chlorine

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    Can't keep chlorine

    I'm giving up and closing the pool this weekend. I have had the same problem all year of not being able to keep chlorine in the pool. I can double shock the pool at night and by morning it's gone. I had my water checked and everything was fine. I finally decided to test for nitrates and the reading was in the 10-20ppm range. I heard if you get a reading at all its too much so I drained half the pool water and refilled it last weekend. Afterwards I tested for nitrates again and got the exact same reading. And yes I tested my tap water which showed zero nitrates. Since then I still have the same problem. I shock the pool and its gone in hours. Here are my latest test results for my 17,000 gallon above ground pool:
    FC- 0.5
    TAC- 0.5
    PH- 7.4
    TA- 70
    Calcium Hardness- 120
    CYA- 35
    Total Dissolved Solids- 600
    Phosphates- 0

    My water is clear but every couple days I can see algae starting to grow again because I can't keep the chlorine up. I bought some product to increase the hardness. Should I increase the TA? What do you recommend when I close the pool? Should I bombard it with bleach and hope for the best? Thanks and good riddance to this season!

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    Join Date
    Mar 2007
    Location
    San Rafael, CA USA
    Posts
    12,082

    Re: Can't keep chlorine

    Welcome to Trouble Free Pool! I am sorry to hear about your frustrating experience. Before you close your pool, can you do one more test? Get an inexpensive ammonia test from a pet/fish/aquarium store and see if you measure any ammonia in your pool water. If present, this will create an insatiable demand for chlorine, though usually will consume chlorine very quickly.

    Nitrates and phosphates just allow algae to grow as they are food (nutrients) for algae, but chlorine alone can keep the algae at bay. I have 2000-3000 ppb phosphates in my pool, for example, but prevent algae growth using chlorine alone.

    Are you having a pool store test your numbers or are you testing them yourself? If the latter, what test kit are you using? It's very important to have accurate numbers, especially for FC and CYA, in order to know what to do and we've seen many inaccurate results from pool stores. The recommended test kit is either the Taylor K-2006 you can get at a good online price here or the TF100 test kit from tftestkits.com here with the latter kit having 36% more volume of reagents so is comparably priced "per test" (and Dave's service is second to none!).

    If you haven't done so already, I suggest you read this article in The Pool School on defeating algae. Shocking a pool at sufficiently high chlorine levels and then maintaining a high enough chlorine level for the CYA level usually prevents algae growth.

    As for the TA and CH, what kind of pool do you have? Is it vinyl, fiberglass or plaster/gunite?

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Re: Can't keep chlorine

    I have a 17,000 vinyl pool. I had a pet store test my water for amonia and it registered zero. As I said it did show 10-20ppm nitrates which I tried unsuccessfully to fix with the partial drain. While I don't yet have a good test kit I've been bringing my water to pool stores to test so the results I gave you are accurate. I put another gallon of bleach in last night plus a bag of dry shock which briefly brought the chlorine level up but by this morning it was registering zero again. I have been putting 1-2 gallons of bleach a day in the pool for the past couple weeks and it's eaten up quickly. At this point I'm tired of pouring more money into the pool and just want to close it. How much bleach do you think I should put in the pool before I put the cover on and do you think it will hold? If I put enough bleach in can I overcome the nitrate problem or will it just eat up as much as I put in? As for the TA do I raise this with baking soda? How much? I put 15lbs of product in last night to increase the hardness. Thank you for your help.

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    Join Date
    Mar 2007
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    Re: Can't keep chlorine

    Calcium Hardness (CH)
    Because you have a vinyl pool and assuming you don't have anything fancy such as tile with exposed grout, then you don't need to worry about your CH and the 120 ppm is fine. Of course, you've already bought and added more which isn't a problem but was not necessary. 15 pounds of calcium chloride (anhydrous) would raise the CH by 95 ppm in your 17,000 gallon pool, so should be about 215 ppm now which is no problem.

    Total Alkalinity (TA)
    As for the TA, you can use The Pool Calculator to calculate quantities though 70 ppm is not bad unless you were to use Trichlor as your source of chlorine. For bleach or chlorinating liquid (or even Cal-Hypo), a lower TA will help reduce the rate of pH rise. It takes about 4 cups of Arm & Hammer Baking Soda to raise the TA by 10 ppm in your 17,000 gallon pool. You can add this amount to get to 80 ppm.

    Free Chlorine (FC) and Defeating Algae
    One gallon of 6% bleach in 17,000 gallons only raises the FC by 3.6 ppm and even two gallons would be 7.3 ppm which isn't anywhere near what is needed to completely kill algae growth with an existing bloom. Bleach usually comes in 96-ounce jugs, not in gallons, and 1-2 jugs would raise the FC by 2.7 to 5.5 ppm. Algae grows and consumes chlorine even when it's not visible -- by the time it's visible the amount of algae is large enough to clump and reflect light. Before that the water could be cloudy, dull or even clear but still consuming chlorine. The way to get ahead of this is to shock the pool to kill all the algae by following the procedure described in The Pool School here.

    Pool stores do not always measure accurately and this is especially true with the CYA test that is critical to know. So IF your 35 ppm CYA level is accurate, then it would take 14 ppm FC consistently held to shock the pool and I'd just play it safe and use 16 ppm FC as a target. That means a LOT more chlorine than you've been adding -- it's about 4-1/2 gallons of 6% bleach or almost 6 96-ounce jugs and that's just to get the FC target -- it takes more chlorine to keep the FC level there until the algae is all killed. If there is a pool or hardware store selling chlorinating liquid at a decent price, then that would be less to carry. Bleach at $1.25 for a 96-ounce jug is equivalent to $3.38 for one gallon of 12.5% chlorinating liquid. I would lower your pH to around 7.2 by adding 2 cups of full-strenght (31.45%) Muriatic Acid (slowly over a return flow with the pump running) before shocking with a lot of chlorine. Chlorine should also be added slowly over a return flow with the pump running. In both cases, for extra safety, lightly brush the side and bottom of the vinyl pool where you've added chemicals to ensure thorough mixing.

    Algae growth is one of those things that you have to get AHEAD of in order to get rid of it and then you need to MAINTAIN an FC level to keep it from coming back. In your case, I'd never let the FC level drop below 3 ppm once you've gotten rid of the algae. You'll know when you can lower the shock level of FC when three things occur: 1) you measure <= 0.5 ppm Combined Chlorine (CC), 2) you measure <= 1 ppm drop in Free Chlorine (FC) overnight, and 3) the pool water is crystal clear. The accurate FC and CC readings are one reason why a good test kit is the first and most important requirement.

    Closing a Pool
    Since you will be closing the pool you have other options. One is to get rid of the algae and be able to hold chlorine as described above and in the link to defeating algae. Then you would need to maintain chlorine levels in the pool over the winter. You didn't say whether it freezes where you live so it may not be practical maintaining chlorine levels over the winter. Some tips for closing an above-ground pool are given here. Another option is to just let the pool go and clean up the inevitable algae mess in the spring. Though this sounds bad, it is something that can be done using bleach or chlorinating liquid alone as shown here.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Re: Can't keep chlorine

    Thank you Richard,

    I wish I had contacted you before paying for the CH product I didn't need to worry about. You are the first person who could tell me that I needed to attempt to overcome the problem by adding large amounts of shock. I was afraid whatever was eating up the chlorine would just continue to no matter how much I put in. I just bought 4 jugs of 12.5% liquid shock. It sounds like this may get me to the 14% level but I'll need more to maintain it. I will try to keep the chlorine level up (although I'm going away for the weekend) and then dump a bunch of shock in next week before I put the cover on. I have a feeling I'll again be dealing with this in the spring. Usually putting the cover on is depressing. This year I'm throwing a party! Thanks for your help.

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    Join Date
    Mar 2007
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    Re: Can't keep chlorine

    It will take about 2.2 gallons of 12.5% chlorinating liquid to add 16 ppm Free Chlorine (FC) to the pool so you are in pretty good shape. After you add it, you need to test again relatively soon and add more if needed, but you'll need the good test kit in order to do that. It's actually not hard to get rid of algae once you understand that you need to hit it hard to get ahead of its growth and then maintain the proper amount to prevent it from coming back.

    There are lots of people here on this forum to help you through whatever comes up in the spring. Check in before closing for some more tips -- mostly using some PolyQuat 60 algaecide after you've shocked the pool since that can help keep it in good shape -- also, wait as long as you can before closing since algae grows much more slowly in cold water.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Join Date
    May 2008
    Location
    South Jersey
    Posts
    65

    Re: Can't keep chlorine

    Richard's advice above is probably the best you can get.

    I went through the same thing with algae in the spring and once I understood that putting in a gallon or two at a time into my 26k gallon pool was really doing very little if anything to kill the algea, things got better much quicker.

    It took me a couple of days of very large doses of chlorine an my pool was back to "normal" chlorine consumption. BTW - The majority of the time I went through this my water was crystal clear. It was only based on the FC and CC test results that I knew I had licked it.

    Good luck!

    Sean
    17x35 Inground Vinyl Liner
    26,000 Gallons
    Nautilus DE Filter
    Pacfab Challenger Pump

  8. Back To Top    #8

    Join Date
    Mar 2007
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    Re: Can't keep chlorine

    A few more points to make about fighting algae. In addition to maintaining a shock level of Free Chlorine (FC), you also need to brush the sides and bottom of the pool to break up any biofilms and expose the algae underneath to chlorine. You also keep the pump running to maximize filtration of dead algae and will need to clean (or backwash, as appropriate) the filter perhaps more than once depending on how much algae you have. You also want to make sure that you pour any concentrated chemicals, including chlorine, slowly over a return flow at the deep end with the pump running and then for extra safety, brush the side and bottom where you've added the chemical to ensure thorough mixing.

    The point Sean made about keeping the FC level up even when the water is clear is very important as algae can still exist and grow back yet not be visible. That's why I gave that rule of 3 things having to occur to know when you are done and can then lower the FC level back to normal levels based on this chart.

    As you can tell by now, it's a lot easier to prevent algae than it is to fight an existing algae bloom. So regular testing and maintaining an appropriate FC relative to CYA is the long-term way of keeping a pool in great shape.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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