Black Algae Is a Surface Problem, Not a Dosing Problem
Black algae is the one that survives a correct dose. You shock the pool, the water tests clean, and the same dark spots are still sitting in the same corner of the plaster a week later. That is not a failure of arithmetic. The chlorine never reached the part of the plant that matters.
It is not a stain, and the test is mechanical
Black algae shows up as small dark spots, blue black or very dark green, dotted on plaster rather than spread across it. It favours rough patches, grout lines, the shaded end and anywhere circulation is poor, which is usually steps, corners and behind the ladder.
The test is touch rather than sight. A metal stain is flush with the surface, and a brush passes over it without catching. Black algae stands proud of the plaster, catches a thumbnail, and leaves a paler ring around the spot once you have worked at it. If nothing you do changes the mark, stop treating it as algae.
Why chlorine on its own slides off
Two things separate this from green or mustard algae, and neither is a question of dose.
The first is the layer on top. Black algae grows in a stack, and the outer part of that stack shields the living tissue underneath from whatever is in the water. Chlorine oxidises the shield and then runs out of steam before it gets to the part that regrows.
The second is that it anchors rather than settles. The roots go into the pore structure of plaster, gunite and grout, so what you can see on the wall is the top of something with a hold on the surface. A larger dose poured into an unbrushed pool mostly buys you a larger chlorine bill.
That is why the order here is not the usual order. Brush first, then chlorinate, then filter, and expect to do all three more than once.
Brush first, with the right brush for your surface
The brush is doing the real work, and the wrong one either does nothing or damages the pool.
| Surface | Brush | Why |
|---|---|---|
| Plaster, gunite, concrete | Stainless steel algae brush | The surface is harder than the bristles, so the heads open and the plaster survives |
| Vinyl liner | Nylon bristles only | Steel bristles are harder than the liner and will cut it |
| Fiberglass | Nylon bristles only | Steel bristles scratch the gelcoat, and scratches are where it grows back |
Work the spots directly rather than sweeping the whole wall, and keep going until the raised head is gone and the mark has faded rather than merely lifted. Then brush the rest of the pool anyway, because the spots you can see are the ones that got established first.
What to add, and how much
Our pool chlorine calculator works from the quantity of each product needed to raise free chlorine by 1 ppm in 10,000 gallons, then scales that to the volume you enter. For a 20,000 gallon pool moving from 1 ppm to 4 ppm, which is the calculator's own default pair of readings, it returns:
| Product | Amount for that pool |
|---|---|
| Liquid chlorine, 12.5% | 61.2 fl oz, about 0.48 gallons |
| Household bleach, 10% | 76.8 fl oz, about 0.60 gallons |
| Cal-hypo granules, 67% | 12.0 oz by weight, about 0.75 lbs |
| Dichlor granules, 56% | 14.4 oz by weight, about 0.90 lbs |
Every extra ppm on the same pool is another 20.4 fl oz of the 12.5% liquid, or another 4.0 oz by weight of cal-hypo. A higher target is that figure multiplied, not a different method, which is worth knowing because this is a treatment you will probably repeat.
What target to aim for is not a number this page will invent for you. It depends on your stabilizer reading, since cyanuric acid holds chlorine back as well as shielding it from sunlight, and on the label of the product in your hand. If stabilizer is something you have never tested, read what cyanuric acid actually does before you pick a number. Note too that dichlor adds stabilizer every time you use it, so it is a poor choice for a treatment you expect to repeat over several weeks.
All of it assumes the volume you typed in is real. If yours is a guess off the back of a pool store receipt, the pool volume calculator derives it from your dimensions, and an estimate that is out by a fifth makes every dose you ever add out by a fifth with it.
Then filter, and brush the same spots again tomorrow
Leave the pump running continuously rather than on its normal schedule until the water is clear, because the filter is what removes what you knocked off the wall. Water that clouds after a heavy brush and a dose is the treatment working. If it stays cloudy once the spots are gone, our guide to clearing cloudy pool water is about circulation and filtration rather than chemistry, which is where that answer normally sits.
Then go back to the same spots the next day and brush them again. One pass rarely finishes a colony, and the second and third passes are what stop it re-establishing from what is left in the pore.
When it is the plaster, not the chemistry
If the spots keep returning in the identical places after several rounds of brushing and correct chlorine, the surface is the reason. Etched, crazed or worn plaster is a texture the roots can hold on to, and no amount of chlorine turns a rough surface into a smooth one. At that point the question stops being a chemistry question and becomes one about the condition of the shell, which is a conversation with a contractor rather than another jug of liquid chlorine.
Scattered new spots in new places are the opposite signal. That points at circulation, at sanitizer that has been running low, or at something that went back into the water carrying it, and those are all fixable without touching the surface.
Frequently asked questions
How do I tell black algae from a metal stain?
Feel it rather than look at it. A metal stain is flush with the surface and a brush passes straight over it. Black algae sits proud of the plaster, with a head you can catch a thumbnail or a brush corner on, and working at it leaves a lighter ring where the surface underneath has been exposed. If a stiff brush changes nothing at all and the mark is perfectly smooth, you are looking at a stain, and that is a different job with different chemicals.
Can I use a steel brush on a vinyl liner?
No. A stainless steel brush is for plaster, gunite and concrete, where the surface is harder than the bristles. On a vinyl liner or a fiberglass shell the bristles are harder than the surface, so the brush cuts the liner or scratches the gelcoat, and a torn liner is a much larger repair than the algae was. Use nylon bristles on those surfaces and accept that it takes more passes.
Why do the spots come back in exactly the same places?
Because the roots were never removed, and they are anchored where the surface is porous. Rough, etched or crazed plaster gives them somewhere to hold that a brush cannot fully reach. Regrowth scattered in new places points at circulation or sanitizer level; regrowth in the identical spots points at the surface itself, and no dosing schedule fixes a surface.
Do I need a black algae algaecide?
It is a follow-up, not a shortcut. Brushing opens the heads, chlorine kills what is exposed, and the filter clears what comes off. An algaecide added to an unbrushed pool is being asked to do the brush's job. If you use one, follow its label, including whatever wait it specifies after shocking and before swimming.