Black Algae?

[HotTexas]

I'm not sure if I have black algae or not. If someone could tell me how to post a pic on here, then I could show you what I have.

I've explained it to the pool supply stores but they just try to sell me chemicals. (I guess because I'm female/or that's their "real" job)

It is not slimy. I've scrubbed it with a hard brush. It just looks like a round brownish stain on my Diamond Brite. They are on the sides, steps, bottom, tan ledge. I don't have any trees, bushes, etc. around the pool to drop things in.

Any help would greatly be appreciated.

[PDub]

Yes thier job is to sell you chems. Did they ask you for a sample of your water to test with? Have you tested your water yourself?

[HotTexas]

Yes thier job is to sell you chems. Did they ask you for a sample of your water to test with? Have you tested your water yourself?

Yes, they tested it and everything is fine. Chlorine is a little high from when I shocked it.

[HotTexas]

Here is a close up pic of one of the spots.

[chem geek]

That looks more like a metal stain to me. Black algae usually has concentrated filament-like (going into the pool surface) areas of dark black small spots which can form clumps if it spreads and isn't usually as diffuse as in your picture. An example of black algae that I found is this crescent one which is pretty far along (i.e. not just new growth).

If this were black algae, then scraping a Trichlor puck on it and holding the puck to it for a number of minutes will have it fade -- this is also true if it were some kind of organic stain (though that's usually more colored such as tannins in pine/cedar needles).

If it's a metal stain, then an ascorbic acid treatment along with a metal sequestrant should work. You can have your pool tested for metals (copper, iron) as that would also help you determine this, though if the pH went way up at some point then you could have precipitated (stained) out most of the metal so that it won't register in a subsequent test.

Richard

[HotTexas]

Thanks for the advice. I had the water checked today and the phosphates were at 200. Also, I have more spots.

1. What causes phosphates in the water?

2. Are there certain plants that should not be around a pool?

Also, someone recommended Granular 90 to try. Any advice?

Thanks again.

[chem geek]

200 ppb is not very high. You can prevent algae in a pool with phosphate levels up to around 3000 ppb or so if you always maintain a minimum Free Chlorine (FC) that is not lower than 7.5% of the Cyanuric Acid (CYA) level.

Phosphates come in from fill water and from soil. Phosphates and nitrates are essential plant nutrients and algae is a form of plant. I wouldn't worry about it.

As for the stains, get your water tested for metals.

As for Granular 90, that is Trichlor that has been ground up. Remember the following rules regarding chlorine products:

For every 10 ppm FC added by Trichlor, it also increases CYA by 6 ppm.

For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.

For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.

If you use Trichlor or Dichlor regularly, the CYA will tend to rise and you will need a higher FC level to keep algae from growing unless you use a supplemental algaecide (PolyQuat 60 or a phosphate remover).

Richard

[HotTexas]

That looks more like a metal stain to me. Black algae usually has concentrated filament-like (going into the pool surface) areas of dark black small spots which can form clumps if it spreads and isn't usually as diffuse as in your picture. An example of black algae that I found is this crescent one which is pretty far along (i.e. not just new growth).

If this were black algae, then scraping a Trichlor puck on it and holding the puck to it for a number of minutes will have it fade -- this is also true if it were some kind of organic stain (though that's usually more colored such as tannins in pine/cedar needles).

If it's a metal stain, then an ascorbic acid treatment along with a metal sequestrant should work. You can have your pool tested for metals (copper, iron) as that would also help you determine this, though if the pH went way up at some point then you could have precipitated (stained) out most of the metal so that it won't register in a subsequent test.

Richard

Where would the metal stain come from? Rebar coming through? Something that was in the mixer when they plastered it? The pool was plastered in Dec.

[chem geek]

Metals usually come from the fill water, especially if it's well water. The other source would be metal in the pool IF it got exposed to acidic conditions. This can happen locally if you have a Trichlor feeder that "parks" itself near some metal since Trichlor is highly acidic. I had that happen in my own pool where the mounts for stainless steel bars nearest to the Trichlor feeder got corroded. Pouring acid in the skimmer is an absolute no-no and can result in corrosion of the copper in a heat exchanger (as well as damaging pump seals and the filter material, depending on filter type).

If there is exposed steel (i.e. not stainless steel) then that will usually corrode in pool water conditions so yes, if rebar poked through then it could corrode, but you should see that and have a stain near that area. Your stains didn't look like that -- rebar would show a reddish brown rust color. When the metal gets oxidized and forms an oxide or hydroxide that stains again, then it can be brown to black.