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

Black or very dark green dots that reappear a day after you brush them are, in almost every case, black algae. It is the one pool problem where the usual advice, brush and shock, genuinely does not work.

What you are actually looking at

Black algae is not a surface film. It anchors into the pores of plaster and grows a layered protective cap over the colony. That cap is the reason it survives chlorine levels that kill everything else in the pool: the sanitiser never reaches the living tissue underneath.

Brushing takes off the cap. The roots are still in the plaster, and within a few days the spot is back in exactly the same place, which is the diagnostic. A stain does not come back.

Where it takes hold first: rough patches, the shaded side, steps and corners where circulation is weakest, and anywhere the plaster has started to go porous with age. A smooth, well-circulated, well-maintained pool is a hard surface for it to establish on.

The honest treatment ladder

If it is new and confined to a few spots, it is worth fighting chemically before spending money:

1. Brush the spots hard with a stainless brush to break the caps open. This is the step people skip, and without it nothing else works.
2. Take chlorine high and hold it, for days rather than hours.
3. Use a targeted algaecide on the open spots.
4. Brush again daily. If the spots are shrinking after a week, keep going.

If it is established across the surface, or it has beaten a full chemical round, the calculus changes. You are not trying to kill something on the plaster, you are trying to remove the layer of plaster it lives in. That is what an acid wash does: a drain, a muriatic acid strip of the surface, a chlorine bath, and a refill.

The thing nobody tells you about acid washing

Acid washing strips a thin skin of plaster from the shell. That is how it works, and it is why it defeats rooted algae and staining that chemicals cannot touch.

It also means a pool has a limited number of acid washes in its life, commonly cited as around five, after which the plaster is too thin and the answer is resurfacing rather than washing. A pool that has been acid washed three times already and is going porous is a pool heading for a replaster, and an honest pro will tell you that rather than sell you a fourth wash that buys eighteen months.

So: fight a young infestation with chemicals. Wash an established one. And if the plaster is already rough and chalky, read the failing plaster page first, because acid will make a thin surface thinner.

Why Mesa pools get it

Black algae arrives on something: a swimsuit or toy from a lake or another pool, most often. What decides whether it establishes is the surface and the circulation. Long hot seasons, high bather load, and plaster that has aged in hard water all help it. So does a pool that spent a summer green, because that is when the plaster surface gets etched and porous enough to hold it.

What it costs

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

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Photograph the spots close up and from the deck, and note how old the pool is and whether it has been acid washed before. That last question decides more than the photos do. We cover Mesa, Gilbert, Chandler, Tempe, Apache Junction, Queen Creek, and Gold Canyon.