Black spots that will not brush off: black algae in the plaster

Dark dots that reappear a day after you brush them are, almost always, black algae. It is the one pool problem where brush-and-shock genuinely does not work, and understanding why saves a season of wasted product.

What you are looking at

Black algae is not a surface film. It anchors into the pores of plaster and grows a layered protective cap over itself. That cap is why it shrugs off chlorine that kills everything else in the pool: the sanitiser never reaches the living tissue underneath.

Brushing removes the cap. The roots stay, and the spot is back within days in exactly the same place. That recurrence is the diagnostic, because a stain does not come back.

Where it takes hold: rough patches, the shaded side, steps and corners where circulation is weakest, and anywhere plaster has gone porous with age. Which is why it is common on older pools and rare on well-maintained newer ones.

The treatment ladder

Young, a few spots. Worth fighting chemically before spending money.

Brush hard with a stainless brush to break the caps open. This is the step people skip, and nothing else works without it.
Take chlorine high and hold it for days.
Apply a targeted algaecide to the opened spots.
Brush daily. Shrinking after a week means keep going.

Established across the surface. The calculation changes. You are no longer killing something on the plaster, you are removing the layer of plaster it lives in, which is what an acid wash does.

The older-pool complication

On a Chandler pool this is where it gets specific, because the acid wash has a cost beyond the invoice.

Every wash removes a thin layer of plaster and a pool has a finite number in its life, commonly cited as about five. On a house you did not build, with owners you never met, nobody knows how many it has already had.

So before you agree to have a pool stripped for black algae, look at the surface: does it chalk on your hand, is it uniformly rough, is aggregate showing. If it is, the black algae is a symptom of a porous dying surface rather than a problem to be washed away, and the honest answer is resurfacing. The rough and chalky plaster page is the test.

What it costs

Situation	What it needs	Price
A few young spots	Brush, chlorine, algaecide	Your chemicals
Widespread, plaster sound	Drain and acid wash	$300 to $800
Large or severe	Large acid wash	$700 to $1,200+
Plaster already thin and chalking	Resurfacing conversation	Prep quoted

From the pricing page; the cost calculator gives your range.

Get a quote

Photograph the spots close up and from the deck, and tell us the pool’s age and anything you know about previous acid washes. That last answer decides more than the photographs. We cover Chandler, Gilbert, Sun Lakes, Ahwatukee, Tempe, and Ocotillo.

Frequently Asked Questions

How do I know it is black algae and not a stain?

Black algae has a raised, slightly crusty head you can feel, grows in defined dots or clusters rather than sheets, and comes back in the same spot a few days after brushing. A stain is flat, smooth, and stays gone once it is properly treated.

Why does brushing not work?

Because you are only removing the cap. Black algae roots into the pores of the plaster and grows a layered protective shell over the colony, which is why it survives chlorine levels that kill everything else. Brushing takes the cap off; the roots regrow it within days.

Can chemicals beat it?

On a young infestation, yes, with stainless brushing to break the caps open, high chlorine held for days rather than hours, and a targeted algaecide. Once it is established across the surface, you are trying to kill something living inside the plaster, and removing that layer is usually the only reliable answer.

What does removal cost?

A drain and acid wash runs $300 to $800 on a typical Chandler pool, or $700 to $1,200 and up on a large or badly affected one. On an older pool, ask for a plaster thickness check first, because a surface that is already thin cannot afford the wash.