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Thread: Algae after hard rain even though FC was 7.5

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    Algae after hard rain even though FC was 7.5

    I returned from a week's vacation late Tuesday night and checked the pool first thing Wednesday morning and everything was it should be, FC 8, PH was 75. I had stacked 8 3" tabs in the skimmer and they were just about gone. The pool was clear with no signs of algae anywhere.

    It started raining hard later in the morning. By late afternoon when I got a break in the rain I went out and checked the pool and I'm glad I did because the water was almost up to the top of my overflow because it was partially blocked by rose petals that had blown in from a climbing rose bush. Since the storm was supposed to last into the night I added another 3.5 ppm of bleach in case I lost power for a day or 2. At this time the pool was clear as a bell. My timer turns off the pump at 7 PM and back on at 6 a.m.

    It continued to rain hard until sometime during the night. On Thursday morning the rain had stopped and my wife came in and said the pool was cloudy. It was mostly visible in teh deep end, similar to the morning after a pool party with lots of kids. I checked the chlorine and it was still 7.5, easily within recommended levels for my CYA which I then checked to be somewhere between 60 and 70 (I wish the black dot test weren't so subjective) which was higher than I like but understandable after my second use of CYA adding pucks this spring.

    I began shocking at a 20 PPM level late that morning and to keep it short, my first overnight test lost 3 PPM but this a. m. test showed no FC drop, the algae is gone, and the water is clear.

    I had a similar but worse problem several years ago when I went to bed with a clear pool. During the evening we got the rain remnants from a hurricane and the next morning my pool water looked like milk. I posted about it then but there were no causal answers. So here are my questions:

    Why doesn't the recommended FC level ALWAYS prevent algae? Can there be so much algae in a storm that it gets overloaded?

    Would it matter that the pump was off overnight? Since as far as I know the chlorine doesn't fall out of suspension so don't know why it would mater. I don't want to leave it on overnight just in case the overflow got blocked by leaves, twigs, rose petals etc.. Water behind the liner would sure be worse than algae.

    Would keeping Polyquat in the pool prevent such a surprise? I'm especially concerned of something like that happening when I was away. I also now realize that my overflow could get clogged while I was gone although that was the first time in 37 years it had almost happened. What a mess that would leave.

    Thanks for any help or ideas.
    25,000 gallon inground vinyl sand filter 60 gpm 1 hp pump

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    Re: Algae after hard rain even though FC was 7.5

    As it turns out, I live North of Falls Lake (in Raleigh) so experienced the same rain as you but with differing results.

    I keep my pool about 50 ppm CYA and FC around 4-7 ppm losing normally about 2.5 ppm daily. I experienced some dilution of all chemistry because the 6.5" of rain we got forced me to drain about 5" out of my pool....about 13% of my total water.
    Why doesn't the recommended FC level ALWAYS prevent algae?
    The short answer is there are just way too many variables. I wish I could explain why you got algae and I didn't but I can only default back to the "too many variables" explanation which is no explanation at all.

    I will say that developing algae is seldom visible and has usually been in a pool growing before you can ever see it.

    How to prevent? Polyquat might help, I have never used it. Probably the best preventative you can use is to elevate your FC in anticipation of rain. That said, I have never had rain have much effect on my pool water chemistry so I have always considered it a non-event.

    My pool is nine years old and rain has virtually no effect on it. I have to believe that most pools are the same so I cannot provide you a really good answer to what you experienced.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Algae after hard rain even though FC was 7.5

    Quote Originally Posted by duraleigh View Post

    I keep my pool about 50 ppm CYA and FC around 4-7 ppm losing normally about 2.5 ppm daily. I experienced some dilution of all chemistry because the 6.5" of rain we got forced me to drain about 5" out of my pool....about 13% of my total water.
    Thanks for the reply although I am now leery of taking trips in what seems to be a time of frequent severe weather patterns. I think I'll try using Polyquat. It's a lot cheaper than shocking to get rid of algae.

    I'm curious about how you were able to hold 5" above normal. Don't you have an overflow or worry about water getting behind the liner or do you have a concrete pool? Regarding algae, from your description it sounds like your pool is about 3 feet deep but I don't see why that would matter when it comes to algae.

    PS I also noticed that I had some dead algae floating on top of my fish pond that morning after the rain. It has a pond algaecide in it.
    25,000 gallon inground vinyl sand filter 60 gpm 1 hp pump

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    Richard320's Avatar
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    Re: Algae after hard rain even though FC was 7.5

    Any chance you had any runoff from the lawn get into the pool? Lawn fertilizer will feed algae and use chlorine like nobody's business.
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
    Troublefree does not mean Maintenancefree. It's like brushing your teeth: You can spend a couple minutes a day and pennies a week or go to the dentist once a year and spend several thousand dollars.
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    Re: Algae after hard rain even though FC was 7.5

    I'm curious about how you were able to hold 5" above normal.
    I pre-drained about 4" from the pool in anticipation of the rain
    .from your description it sounds like your pool is about 3 feet deep
    I don't understand that. The pool averages 4' in depth.

    My pool is concrete/vinyl and the liner stays perpetually submerged. Little if any water gets behind the liner.

    Polyquat may work well for you. I haven't used it so cannot give you an educated opinion.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Algae after hard rain even though FC was 7.5

    Quote Originally Posted by Richard320 View Post
    Any chance you had any runoff from the lawn get into the pool? Lawn fertilizer will feed algae and use chlorine like nobody's business.
    Not likely since the only thing at pool level is a flower garden on one side of the pool separated by a 3' concrete walk that slopes back toward the flowers. It gets minimal fertilizer and hasn't had any recently.

    It is something I hadn't thought of though. Also my chlorine was still in the correct range after the pool became cloudy.
    25,000 gallon inground vinyl sand filter 60 gpm 1 hp pump

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    Re: Algae after hard rain even though FC was 7.5

    Quote Originally Posted by duraleigh View Post

    My pool is concrete/vinyl and the liner stays perpetually submerged. Little if any water gets behind the liner.
    That is interesting. I had always heard that getting water behind the liner was a disaster and that is why they put in an overflow. Is it not a problem for you because you have concrete behind your liner? There are posts at the site that discuss getting water behind the liner and how to get it out. Apparently I don't understand why it is or isn't a problem.
    25,000 gallon inground vinyl sand filter 60 gpm 1 hp pump

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    Re: Algae after hard rain even though FC was 7.5

    I don't get any water that collects behind the liner because I have a French drain around the perimeter of the bottom of the pool. Any water that seeps from the ground surface quickly goes to the French Drain and gravity carries it downhill to daylight.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Algae after hard rain even though FC was 7.5

    Quote Originally Posted by bobjdan View Post
    I returned from a week's vacation late Tuesday night and checked the pool first thing Wednesday morning and everything was it should be, FC 8, PH was 75. I had stacked 8 3" tabs in the skimmer and they were just about gone. The pool was clear with no signs of algae anywhere.

    It started raining hard later in the morning. By late afternoon when I got a break in the rain I went out and checked the pool and I'm glad I did because the water was almost up to the top of my overflow because it was partially blocked by rose petals that had blown in from a climbing rose bush. Since the storm was supposed to last into the night I added another 3.5 ppm of bleach in case I lost power for a day or 2. At this time the pool was clear as a bell. My timer turns off the pump at 7 PM and back on at 6 a.m.

    It continued to rain hard until sometime during the night. On Thursday morning the rain had stopped and my wife came in and said the pool was cloudy. It was mostly visible in teh deep end, similar to the morning after a pool party with lots of kids. I checked the chlorine and it was still 7.5, easily within recommended levels for my CYA which I then checked to be somewhere between 60 and 70 (I wish the black dot test weren't so subjective) which was higher than I like but understandable after my second use of CYA adding pucks this spring.

    I began shocking at a 20 PPM level late that morning and to keep it short, my first overnight test lost 3 PPM but this a. m. test showed no FC drop, the algae is gone, and the water is clear.

    I had a similar but worse problem several years ago when I went to bed with a clear pool. During the evening we got the rain remnants from a hurricane and the next morning my pool water looked like milk. I posted about it then but there were no causal answers. So here are my questions:

    Why doesn't the recommended FC level ALWAYS prevent algae? Can there be so much algae in a storm that it gets overloaded?

    Would it matter that the pump was off overnight? Since as far as I know the chlorine doesn't fall out of suspension so don't know why it would mater. I don't want to leave it on overnight just in case the overflow got blocked by leaves, twigs, rose petals etc.. Water behind the liner would sure be worse than algae.

    Would keeping Polyquat in the pool prevent such a surprise? I'm especially concerned of something like that happening when I was away. I also now realize that my overflow could get clogged while I was gone although that was the first time in 37 years it had almost happened. What a mess that would leave.

    Thanks for any help or ideas.
    I had the same thing happen the first year I had shifted over to chlorine. Everyone assured me that my chlorine level must have dropped off or I must have had runoff other then rain get in the pool. I was skeptical since I was keeping a pretty good eye on the chlorine and its an above ground pool with no runoff and no noticeable debris was in the pool.

    As a precaution, the next season I added 2 oz of algaecide weekly and had no problems. This year I skipped the algaecide and low and behold today, after a few inches of rain yesterday, I have a cloudy green pool with a FC level of 5. It always happens late in the season when the pool temperature is in the upper 80s, and when the cover has been on with no swimming activity for a few days or weeks.

    I'm not sure if this has to do with the type of algae in my area, or what, but I was never able to stop the algae with Baquacil either. Thats part of the reason I shifted to chlorine.

    I'm convinced, for my area, I will use weekly algaecide treatments and have a trouble free pool.

    Rick
    24' Round AG Vinyl Pool (13,500 Gallons)
    Sand Filter, Aquabug Vacuum
    Pump Size: ?? Flow Rate:??
    Shifted from Baquacil to Chlorine 8/2011

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