Black algae can be very difficult to get rid of. I have heard of two approaches. One is to shock the pool for a couple of days and brush the algae as frequently as possible while the chlorine level is high. The other is to try rubbing the algae spots with a trichlor puck for a couple of minutes. In some cases people reported needing to use a higher FC level than usual forever after or the black algae would come back.
Welcome to the forum. Black Algae is also easily misdiagnosed. If you are completely new to pools, don't be 100% convinced that's what you have. Is your pool plaster or vinyl? How was the algae identified?
Here's what little I know about black algae...very similar to Jason's input.
It's fairly rare occuring usually in plaster pools and it's a tough nut to crack. Treatment's that make sense to me generally consist of prolonged exposure (maybe 2-3 weeks) to high concentration of free chlorine.....Not necessarily shock level but say, maybe twice normal. Then, absolutely brush vigorously and daily to expose the algae to the chlorine...perhaps even with a wire brush.
Lastly, chlorine levels may need to be kept higher than normal permanantly to prevent it recurrence.
I have no personal experience but that's a synopsis of what little I've read that makes sense to me....many of the other "cures" I've read about don't.
Here's far better info from, in my opinion, the absolute guru of practical experience with pool water chemistry....Ben Powell. It was posted in response to in inquiry as to how to treat for black algae:
As far as brushing . . . it's important, but persistence is more important than strength. I've seen folks attack it, as if one really, really hard scrubbing would get it all cleaned up. No such luck! If you want to get rid of black algae after it's become established, you have to commit to approaching it as a long term problem. Think weeks, not days.
(Of course, many pool stores will be happy to sell you any number of products that promise quick results. And, they'll also be happy to explain -- when you don't get quick results -- why it didn't work for *you*)
What's important is brushing enough to remove any dead top layer, or at least to scratch it, and expose the live algae to the chemicals in the pool. But it's much more important to brush some today, and the day after and the day after that, than to brush really, really hard today.
Another thing you need to keep in mind: black algae doesn't grow rapidly, and didn't appear in your pool suddenly. It's there because, day after day, the chemical levels weren't right. And, once it's gotten a start, it's unlikely you will EVER get rid of all the roots and spores penetrating into your pool. (I'm assuming it's NOT a vinyl or FG pool??) So, even if you 'get rid of it' for now, it will come right back unless you learn to maintain proper chemistry day after day in the future.
Now, you didn't post test results, or tell how you've been treating your pool, so I don't really know what you've done wrong. There are numerous possibilities; here are a few of the more common:
Low, or inconsistent, chlorination.
High pH, not compensated for with higher chlorine.
High stabilizer, not compensated for with higher chlorine.
Poor circulation, or not running the pumps enough.
Porous or damaged plaster, not compensated for with higher chlorine.
Keep in mind that a single day -- or even week -- of one of these problems wouldn't lead to black algae. It just doesn't grow that fast.
So, what you really need to first, is figure out what's been going wrong, and learn how to correct that. Otherwise, anything you do to get rid of the algae will be only very temporary in effect! Fortunately, as many PoolForum members can tell you, doing it right is easier and cheaper than doing it wrong, once you learn how!
Meanwhile, except for the trip and fall hazard from the black algae, there's no reason to not use the pool while you work on the problem . . . as long as you have chemistry that's adequate for sanitation
Once you figure out what you need to correct, and take care of that, the solution is pretty straight forward:
Maintain chlorine levels 50% higher than you'd otherwise need to maintain.
Using polyquat will help some, especially if you can FREQUENTLY brush hard. But, it's still going to take weeks, as a rule, not days.
You have a salt water chlorine generator and you have black algae? What brand of SWG do you have and what is your CYA level? I have seen several pools with SWGs that developed black algae and in every one there was either very little or no CYA in the water!
Please post a full set of test results for FC, TC (or CC), pH , TA, CH, CYA, and salt (not from the readout on your swg but by a chemical test or calibrated meter) and also tell us the type of SWG, the kind of filter (cartridge, sand,or DE), and the surface of your pool and how many gallons it is.
This info will allow us to determine what is going on in your pool and if you do, in fact, have black spot algae and , if so, how to treat it.