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Thread: Not sure if I have black algae

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    Not sure if I have black algae

    (My first post here!) I have a 15,000 gallon inground gunite salt chlorinated pool, 10 years old, with white plaster. This summer for the 1st time there are tiny black spots mostly on the step surfaces. My water is crystal-clear. I also have greenish stains on the bottom of my pool so I hired a pool maintenance professional to test the greenish stains and said they are a mineral (iron I think.) He thinks the black dots are black algae, and he told me I should get rid of the black algae before he treats the stains. He recommended I buy the best algae killer (not maintenance, but killer) and brush them often and keep my chlorine level up over 5 until the spots are gone. I added the black algae killer - half a bottle, then 2 days later the other half of the bottle, and have kept the clorine level high. That was 2 weeks ago and I don't think any spots have disappeared. When I brush with the wire brush, I brush hard back and forth - no matter how much I brush or how hard I see no difference - nothing comes off as I brush and the next day it looks the same. I touch the black dots I can't feel anything, just plaster - it doesn't feel soft or smooth, and nothing comes off with my fingernail. It looks like someone used a black fine sharpie pen and marked tiny dots in my plaster! To describe, it looks like vanilla ice cream - you know how there are dark spots in vanilla, that's what my plaster on the steps looks like. I'm wondering if I should buy some chlorine pucks or trichlor pucks and see if that does anything to the dots? Maybe it isn't black algae? What do you recommend?

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    Mod Squad JVTrain's Avatar
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    Re: Not sure if I have black algae

    You could try setting a trichlor puck on the stain to see if it disappears, indicating an organic based stain. You can usually buy individually wrapped trichlor tabs most places so you don't have to have leftovers. You can also try a vitamin C tablet or two on the stains as those will take out metal based stains.

    Welcome to TFP!
    Joel - TFP Moderator - Minnesota - **Become a TFP Supporter!** Helpful Links: ABCs of Pool Water Chemistry - SLAM Procedure - Chlorine/CYA Chart
    40x20 Pool: 32K Gallons * Vinyl * Bleach Chlorination * Hayward S270T Sand Filter * Pentair SuperFlo 1 HP * Teledyne/Laars Heater * AquaVac Tigershark * TF-100 w/ SpeedStir
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    duraleigh's Avatar
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    Re: Not sure if I have black algae

    Welcome to the forum.

    Good suggestions from jvtrain.

    Your pool maintenance needs to change. Algae only gets a foothold where the maintenance has been inadequate. We can teach you how to care for your pool and do some precision testing so you don't have that issue again. If you don't change how you are doing things, the issues will come right back.

    Scrape that black algae with your fingernail and, if you get some off the pool surface, smear it on a white sheet of paper.....it will be very dark green if it's black algae.

    I always suggest to newbies that you read "The ABC's of Pool Water Chemistry" up in Pool School
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Re: Not sure if I have black algae

    Thanks for both the replies! I had no idea I could buy a single trichlor tab. I didn't want to buy a 40 lb tub! Also, I have recently changed the way I maintain my pool because got a solar blanket and a new pentair intelliflo pump, so I had a little learning curve and had to make some changes in how I care for the pool and chemicals. The pump is circulating the water much better and longer, and the blanket is keeping the chemicals in. I use a Taylor k2006 test kit. I can't scrape anything off the black spots with my fingernail (they are all pin-point size) - I will try the trichlor puck and see what happens. And I will read the ABC's of Pool Water Chemistry carefully, too!

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    Re: Not sure if I have black algae

    Did you ever find out what the black dots wher? or get rid of the black dots?
    14000 gallon SWG gunite/quartz plaster,450 swimclear cartridge filters, 2hp pump.

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