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Thread: Is this algae?

  1. Back To Top    #1

    Is this algae?

    Does this look like black algae? I have treated it twice in 3 weeks, but it contines to grow/appear. The largest spot is about half the size of a pencil eraser and they are not growing any bigger; most are much smaller. It only grows/appears on the bottom and curved part of the wall, never on the wall itself. I have scrubbed with a steel brush and chlorine tab, then treated with Black Algae Killer from Leslies, twice. My water chemistry has held constant for the past few months, during the time that it started to appear. A test from today is below:

    FAC 3 ppm
    TAC 3
    Salt 4,000 ppm
    CH 250 ppm
    CYA 100 PPM
    TA 100 PPM
    pH 7.6
    Copper 0 ppm
    Iron 0ppm

    The pool is a 10,000 gal saltwater pool, and is 3 years old. I have never had any issue with it until this problem has been creeping up. In the pictures, the small dark (almost black) spots are the 'algae' that I am referring to. There is a bit of mottling in the picture, but the dark spots on the curved part of the wall is what is growing/spreading.

    Thanks in advance,

    Attached Images Attached Images

  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: Is this algae?

    Welcome to TFP!

    Probably yes. It tends to be difficult to tell for sure from a photo. A simple way to confirm it is to scrape off a black spot with your fingernail and spread it on a piece of white paper. If it smudges green on the paper then it is almost certainly black algae. If you can't scrape it off, or it smudges black, then it probably isn't black algae.

    From your numbers I see a problem that might be related to the appearance of the algae. Your CYA level is too high and your FC level is too low. You don't want CYA above 80. Worse, most of the common CYA tests will report any level over 100 as 100, so your CYA level might actually be much higher. Even with a SWG, and even if your CYA level really is only 100, you should be keeping the FC level between 5 and 8, and never below 5. If CYA is higher than 100, the minimum FC level would be even higher.

    Black algae doesn't get started at all easily. It requires an extended period of lower than recommended FC levels. If these levels are representative, that would be enough to allow it to get started.
    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

  3. Back To Top    #3

    Re: Is this algae?

    Thanks Jason. So, should I work on getting the FC up first? Or work on getting the FC up and CYA down, at the same time? I'll see if I can't scrape some of the black spots off tonight and see if they smudge.

  4. Back To Top    #4
    Butterfly's Avatar
    Join Date
    May 2007
    South Carolina

    Re: Is this algae?

    Quote Originally Posted by zhart
    Thanks Jason. So, should I work on getting the FC up first? Or work on getting the FC up and CYA down, at the same time? I'll see if I can't scrape some of the black spots off tonight and see if they smudge.
    Lower the CYA first.
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  5. Back To Top    #5

    Re: Is this algae?

    It would be nice if there were a "Valtrax" for pool herpes, AKA black algae. My experience is antidote and I'm not an expert but my pool does suffer from Black algae and I think I have a handle on it.

    During the winter (short here in Florida) I let the pool go and in the spring, I have little black spots in the pool. I keep TA about 60 and CYA about 50. My pool requires large doses of acid, about a quart a week. (That may not be large but seems like that to me.)

    So, with all other numbers in the recommended range, I'll lower Ph to about 7.2 and shock to about 20ppm. I then done the scuba gear and grab a small pumice stone and knock the all heads off. A scotch bright pad might work. I might try that next time.

    After shocking, I keep FC up about 5-6 with my SWG and it is good for the year. I refuse to use any pool store black algae products that contain metal.

    There are folks here a lot smarted than me but this works in my case. Read/Follow the "Pool School" (link on top of page) and the advice of the experts and all will be well.

    Has anyone ever had luck with an acid wash? I'm not going through the trouble. I have 3 Live Oaks and I think the BA originated from their leaves.
    22,000 Gunite, Cart Filter, Aquarite SWG

  6. Back To Top    #6

    Re: Is this algae?

    I tried to scrape off some of the spots last night, but they are so small that I couldn't collect a sample to see if it smudged. They did scrape off though.

    Frank, I too have large oak trees, maybe that is where mine came from as well. I'm planning on knocking the heads off with a pumice stone, scrubbing with a chlorine tab, then treating again. Ready for these spots to be gone so I can enjoy using the pool and not having to worry about or plan my next move against this black algae!

    BTW, like your pool, mine requires large doses of acid. My neighbor and I had our pools put in, and filled, at the same time and are the same size. His is chlorine, mine is salt water, and mine has always required WAY more acid than his. Don't know if is due to one being salt and the other chlorinated, or what, but I do go thru the acid.

  7. Back To Top    #7

    Re: Is this algae?

    My pool had a large acid demand when it was new, from the plaster curing I was told. The SWG came about 2 years later and yes, it is supposed to constantly raise Ph. There are folks here who know the chemistry, I'm not one of them!
    22,000 Gunite, Cart Filter, Aquarite SWG

  8. Back To Top    #8

    Join Date
    Apr 2012
    Lake Charles, Louisiana

    Re: Is this algae?

    Don't know about the BA, but the Ph rise is normal with a SWG. The SWG aerates the pool, which in turn raises the Ph.
    ~20,500G 18x36 IG Vinyl Sports bottom
    300lb Hayward sand filter 1HP Hayward superpump
    Aquarite SWG T15

  9. Back To Top    #9
    alanpaul's Avatar
    Join Date
    Aug 2010
    South Florida

    Re: Is this algae?

    Question: Is a Salt level of 4,000ppm appropriate for your SWG? Some don't function above 3,500, so what brand is yours, please?
    Alan in Wellington, FL - near West Palm Beach
    15,200 gal IG, painted plaster, 1988; 200 SqFt cartridge filter, 2013; 1.5 HP 1-speed pump (uprated), 2005; heat pump, 2005;
    soon to repair (dead) SWG AquaRite T-15 cell, 2005, and replace (dead) suction-side cleaner, 2005.
    Almost everything died this winter 2012-'13, and I know we shouldn't complain... but, Arrgh!

  10. Back To Top    #10

    Re: Is this algae?

    It is a Pentair. I would be REALLY wary of a SWG that didn't function over 3,500. The preferred salt level is 2,900-3,900. Above 4,000 is gives a warning light letting you know the salt level is starting to get too high.

    The chlorine generator continuously reads the salt level and a gives a waning when it is too high or too low. However, I've tested as high as 5,000 ppm with out it throwing a warning light. During that time I know it was generating chlorine because I my chlorine level never dropped.

    When the salt level gets too low I just dump in a full bag of salt. Sometimes it is too much, but a few summer showers and it is diluted back.

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