Is this just regular algae?

Xentex

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Bronze Supporter
Apr 14, 2018
33
Devon, PA
Pool Size
33000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Jandy Aquapure 1400
This is our sixth season in our house, which has a plaster pool that was basically a swamp when we bought it. It cleaned up pretty well and we haven't had any significant issues, but I do have an algae question somebody may be able to help me on.

I'm familiar with the traditional green algae haze you'll get on the sides, that brushes off easily. That's the stuff I generally think of when I hear the term "green algae."

I see people asking questions about "black algae" from time to time, and understand that's a different and more difficult animal. Similar for "mustard algae."

I'm attaching a picture of what I have. It's dark green spots that appear black from a distance, but if you get up close and scrub them down or scrape them off and inspect them they're green. You can see in the picture, they're generally round and will grow in clusters that are generally roundish.

It tends to grow in places where the plaster is very rough, kind of like medium grit sandpaper. (I need a new plaster job, but that's a separate issue.) The attached photo is on the steps in the low end. The biggest clusters are maybe the size of a pencil eraser.

My question is whether this is just run of the mill algae that I need to scrub off and keep the proper chlorine level, or is this something different (like black algae) that requires some special treatment? (Other than scrubbing it off with a wire brush.)
 

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Your description sounds like black algae. Read Black Algae
That's what I was thinking, but mine doesn't look anywhere near as bad as the example pictures so just wanted to get some perspectives.

If I scrape them off with a fingernail I do feel the "waxy barrier" mentioned in that link. I guess I'll consider myself fortunate that they're not as big and pervasive as in some of those photos. But it sounds like my only remedy is to get in there and scrub scrub scrub. (Which is what I end up doing every time I open and again now and then during the season.)

I'm guessing that I get more of this than is normal because so much of my plaster is like sandpaper that it gives the black algae a lot of great places to grow and hold on.
 
Yup, the worn plaster is a breeding ground for black algae.

@Katodude is our black algae whisperer and advises pick at the the black algae spots with some sort of physical method. Wooden stick, wire brush, or my person favorite, weak pressure washer. Then apply the strongest LC to those spots. Pump sprayer, dump jug over the spot. More than once is better. Then run FC hot.
 
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Yep pretty much Black Algae.

What @ajw22 described is what you need to do. You have to disrupt the black algae. If you have less than 10 spots, use a bamboo bbq skewer. If you have more, a weak pressure wash will make quick work of it. Get in the pool with a mask and snorkel and have at it. I personally dont like the wire brush method. I think it is too aggressive.

Then apply the strongest LC to those spots as you can, either just dump it and let it flow over, or get a pump sprayer. I have gotten lazier over the years and just dump. Sure it uses more chlorine, but you probably need to raise your level anyway.

Run FC at 20% of CYA for at least several weeks.

This is the part that sucks, run FC for the rest of time at a minimum of 12% of CYA, 15% is better.
 
a weak pressure wash will make quick work of it. Get in the pool with a mask and snorkel and have at it.

Interesting idea that I'd never thought of. So just take the wand part of the pressure washer down into the water and pressure wash it off. That will be a whole lot easier than the wire brush method.

I have on the order of 100 spots. All pretty small, but plenty of them. I like that pressure washer idea.
 
Yep put on a mask with a snorkel. Get in the pool with the wand and blast away. You will learn quickly how close you need to get.

The pressure washer will pop each of those out. 100 should not take you too long. Then buy some super strong LC 10% minimum, and have it wash over the affected areas.
 
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