Help - Black Algae in the Pool? What Next

texas_steveJune 1, 2008

I live in Houston, TX and have a grey plaster pool with moss rock/flagstone coping. Over the past several months, I have developed a pretty bad black algae problem.

The black algae is present in over 300+ spots (about the 1/4 dime size) and along the sides of the flagstone tile.

I have gone to several pool stores and have gotten many different treatment plans including copper, trichlor and other chemical treatments. I really don't know who to believe.

My water is relatively clear and my chemicals are is good balance (free chlorine, cynauric acid, Ph, etc.) I don;t think I have been running the filter long enough.

I realize black algae is very strong and requires a lot of scrubbing to pentrate the organism. Can a pool expert please recommend an effective treatment plan including the chemical(s) that work best?

Thanks in advance for your help.

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So you only see the algae on the tile and not in the pool? Is this on the coping are do you have flagstone as tile inside of your pool? I'm having trouble understanding where this is? Maybe a picture would help. And you are sure that this is algae right...?

BTW, I have Arizona flagstone and mine have natural black spots on the flagstone...they get blacker when they are wet. What you have scraps off and is a different color underneath...right. If its been an ongoing problem, you going to want to confirm this is algae and have someone look at it under a microscope...if you can.

    Bookmark   June 2, 2008 at 11:45AM
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I want to come see this. I think I can fix it.

    Bookmark   June 2, 2008 at 1:38PM
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Thanks for the replies.

I am quite sure it is black algae. Most of the spots are on the plaster surface and can be scraped off still leaving a residual green spot and some etching.

On the flagstone tile, there are also green/blue/black spots as well.

Literally within a week, the algae problem (algae blooms/spots) has spread throughout the pool.

Ideas on treatment?

    Bookmark   June 2, 2008 at 2:02PM
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At walmart the sell a small sponge (6x4.5 inch) on a handle. Get a bucket with chlorine bleach and dip it in and use the bleach at strength, around the parameter against the tile.

(You can also use Muriatic acid dilluted 1:4.)

You have to get it all...if you let some will come back.

Once you are done, fill your pool to the brim, chlorinate your pool with 4-5 gallons of chlorine bleach or at least 50% more shock than the package says. Check again in 24-48 hrs and make sure you free chlorine is still at least 4 or greater. If not add more chlorine. Run your filter non stop. Wait for the chlorine to return to swim-safe concentration.

    Bookmark   June 2, 2008 at 3:02PM
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Once the chlorine is high, brush the plaster and all sides of your pool with a brush.

    Bookmark   June 2, 2008 at 3:35PM
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Also keep your pH below 6.8 while you are scrubing with the brush and high makes the chlorine work harder.

    Bookmark   June 2, 2008 at 4:16PM
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Thanks for all of the replies! I am running my pump/chlorine generator on superchlorinate non-stop to try to get ahead of the problem. Brushing like crazy...

Any other good remedies?

    Bookmark   June 2, 2008 at 10:37PM
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You mean superchlorinate on the salt generator???....thats not going to do anything. Forget the SWCG...You need massive amounts of chlorine instantly...4 or 5 gallons of bleach. Add acid so that the pH is 6.8...which makes more Hypochlorite...and makes the pool even more "undesireable". Do all the changes at once so nothing in the pool can adapt to all the changes.

Turn the SWCG off..your not going to need it for a week.

That algae burns on the outside and protects itself on the you need to have so much chlorine in there...that even if its just barely getting to the underside of the algae that its so strong its killing it.

Your not going to hurt your pool or the water. Everything will come back in equilibrium in a week or so.

    Bookmark   June 2, 2008 at 10:48PM
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Below is a product you can find at a pool supply and I'm sure you will be happy with the results...

Here is a link that might be useful: Black Algae

    Bookmark   June 3, 2008 at 7:06AM
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Muddy Water:

I saw a previous post by you recommending this product and I tried to locate it in Houston. Only available at pool distributorships. Homeowner schmucks like me can;t shop there.

Can you tell me what the active ingredient is in Black Treat? I can;t tell from the label.

It looks like I can get it online.


    Bookmark   June 3, 2008 at 9:20AM
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Barco is correct about the SWG, super-chlorinate is not going to get you anywhere close to the shock levels you need to get this under control. Black algae is the worst to get rid of because it roots into the plaster. When you scrub it off basically you're just removing the head, a small part of it remains in the plaster and it just comes right back again when it gets a chance. The key is raising your free chlorine to shock level and holding it there while you brush 3 or 4 times a day with a pool brush. Once it's gone hold the FC level for a day or 2 just to make sure.

You can refer to Ben's Best Guess chart to determine the shock level you need (based on your CYA level):

And refer to Jason's Pool Calc to determine how much bleach you need to add to get there:

Don't buy a bunch of "magic cures" from the pool stores, plain old bleach will do the trick and cost a lot less!

    Bookmark   June 3, 2008 at 10:49AM
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Steve I'm not at my store today so I can't look but Ive used it on pools and it works...Give the people at United Chemical a call they are great people and will also help you locate a source...

    Bookmark   June 3, 2008 at 10:57AM
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Can you tell me what the active ingredient is in Black Treat? I can;t tell from the label.

Probably either copper salts, colloidal silver or poly quats.

Most of the stuff sold to kill don't really want in your pool.

    Bookmark   June 3, 2008 at 12:08PM
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