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Thread: Black algae - progress report and more questions

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    Black algae - progress report and more questions

    We have been trying to get rid of black algae in our white plaster pool since early September caused by having our CYA levels too high and FC levels too low for CYA level. Following the advice we got here we lowered our CYA level by draining and refilling the pool Then we shocked the pool very rigorously (FC 30+) until we stopped losing FC overnight and then let the FC levels drop. We didn't see any visible change in the little black spots we have all over our pool except we stopped getting smears when brushing.

    When the FC got down around 18 we had a renewal of overnight loss so we re-shocked the pool(FC 20+) until no overnight loss.

    All along we have been brushing 1 -2 times daily with SS brush.

    When we dropped the FC level for the 2nd time we got no overnight loss so we have been maintaining the FC level at 8 - 15 ppm. Only exception is that 2 days ago the FC dropped to 6 ppm over the course of an unusually hot day but we raised it back up to 15 ppm that evening. There is no FC loss overnight.

    The weather here in Sacramento has shifted so we have high 70's during the day but the overnight temps are down into the upper 40's. The pool water is pretty darn cold at this point.

    We still have the black spots all over the pool. So we have some questions:

    1 )Do we need to continue brushing the pool daily or is it safe to drop back to a less frequent schedule?

    2) Will this continuous brushing w/SS brush over a long period damage the plaster or wear it down/weaken it?

    3) We understand it can be hard to tell when you've killed the black algae because it can stain the pool. And that it can stay alive in the roots and rebloom from that very easily.

    Assuming these are stains and not active algae spots (given that we aren't having any overnight FC loss) is there anyway to get rid of the stains?

    4) Is there anyway to seal the plaster surface to keep the algae from re-blooming (assuming the roots are still alive)?

    5) We were planning to put a cover on the pool for the winter but we're worried that when we take it off next spring we will find a disaster due to regrowth of the black algae. Is this a realistic concern?

    6) Could some of the spots be stains from something else? Before we got advice here we had used Black Algaecide 3 times on the pool and have a small spot where there is blue staining on one of the steps. We also have some very small rust colored spots on the same step.



    15,000 IG white plaster, Hayward 4 cart filter, Jet Vac sweep

    FC 14
    CC 1.0
    pH 7.5
    TA 120
    CH 175
    CYA 55
    17,000 gal IGP, White plaster, Hayward Swimclear Cartridge Filter system, Sta-Rite Max-e-pro 1/2 hp pump; 3/4 hp booster pump

    Sacramento, CA

  2. Back To Top    #2

    Re: Black algae - progress report and more questions

    As long as you have black spots in the pool you need to keep the FC at about 35 ppm and keep brushing as much as is humanly possible or you will NOT kill the black algae. Overnight FC loss is NOT a major factor when trying to kill black algae. The disappearance of the spots are.

    It is possible that your spots are not black algae but are actually copper stains from the algaecide you used but I tend to think they are not. Blue staining is indicative of copper and rust colored spots could be iron, copper, or scale. Black spots could be copper but it usually produces a more diffuse stain. One possibility is that you had a high copper level and then shocked with cal hypo and the undissolved cal hypo settled on the pool bottom and reacted with the copper to produce the black spotting (but if you have spots ANYWHERE besides the bottom of the pool this theory goes out the window! Also, this would tend to create more pinpoint spots instead of the coin sized spots indicative of black algae.)

  3. Back To Top    #3

    Re: Black algae - progress report and more questions

    Hm, well I'm not sure what to say as your advice differs somewhat from the advice I got from other moderators in the past. Everyone else told me that overnight FC loss was the main sign that we had killed any black algae and that it might leave stains so it would be hard to tell by looking that we had killed it off.

    We do NOT and never have had coin sized spots. The spots are much smaller than that. The smallest spots are pinpoint size up to the largest which might be as big as the tip of a pencil eraser. Nothing that is anywhere close to being as big as even a dime. The spots are on the sides of the pool as well as the bottom so I don't think it could be copper staining either.

    I'm pretty sure that at least some of the spots were black algae because when we brushed them it left a dark smear across the plaster. We can still see the dark smears.

    Also I was shocking at 35 - 40 FC at first and then was told by a couple different moderators that FC levels of 20 or so were good enough.

    17,000 gal IGP, White plaster, Hayward Swimclear Cartridge Filter system, Sta-Rite Max-e-pro 1/2 hp pump; 3/4 hp booster pump

    Sacramento, CA

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    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: Black algae - progress report and more questions

    Overnight FC loss is the most dependable indicator for normal green algae, but it isn't definitive for black algae. With black algae you can kill off most of it, at which point the overnight FC loss goes away, but there can still be more left that can grow back as soon as the FC level goes down. The key, as waterbear said, is very frequent brushing.

    Black algae grows a thick protective layer that shields it from high FC levels. Brushing usually only removes a small part of the protective layer, so the chlorine can work on the next portion down. Brushing frequently helps you make progress and will eventually get you down to the roots.

    I'm sorry there was confusion previously. When reading through many many messages we can sometimes get confused and give you the solution to a problem you don't have, instead of the one you actually have.

    As long as the plaster is at least one year old, brushing shouldn't cause any problems (even frequent brushing).
    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

  5. Back To Top    #5

    Re: Black algae - progress report and more questions

    Thanks for clarifying about the FC loss overnight. I guess we need to go back to shocking the pool then.

    Is there consensus on how high the FC level should be for treating black algae?

    I've been told 35+ is required and I've also been told 20 is high enough - that's a big difference. Does the shock FC level change depending on how hot it is during the day?

    We had our pool re-plastered in 5/07 so we should be OK. Part of the reason we wanted to know about that was because when we clean the filters we are washing out a fair amount of what we think is plaster dust.

    17,000 gal IGP, White plaster, Hayward Swimclear Cartridge Filter system, Sta-Rite Max-e-pro 1/2 hp pump; 3/4 hp booster pump

    Sacramento, CA

  6. Back To Top    #6

    Re: Black algae - progress report and more questions

    Black algae takes much higher FC levels to kill it than green algae. 35+ is what I would do.

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