Black algae versus green algae, and why they are not the same fight

What is black algae actually?

It is not really algae in the way green algae is. Black algae is a cyanobacteria, an older and tougher organism, and it behaves nothing like the green bloom most owners have seen. It grows in tight colonies that anchor into the pores of plaster and grout rather than drifting in the water.

Each colony builds a waxy protective layer over itself. That layer is the whole problem. Chlorine in the water passes over the top of it and never reaches the living cells underneath, which is why an owner can shock a pool to a very high level for a week and watch the black spots sit there unbothered.

It also puts down what amount to roots into the surface. Kill the top and leave the base and the spot regrows from the same place, in the same shape, usually within a couple of weeks. That is the signature: a spot that returns exactly where it was.

How do you tell black algae from a stain?

Feel it first. Black algae sits proud of the surface. Run a fingertip over the spot and you will feel a small raised bump, sometimes with a slightly slick cap. A stain is flat, and your finger crosses it without noticing.

Then test it with chlorine. Hold a chlorine tablet against the spot for thirty seconds and lift it. If the spot lightens noticeably, it is organic, either algae or a leaf stain. If nothing changes, you are probably looking at a metal stain, and that is a completely different guide and a completely different treatment.

Then look at the pattern. Black algae favors rough surfaces, plaster imperfections, grout lines, the shady side, steps, and anywhere circulation is poor. It shows up as discrete round spots the size of a pea up to a dime rather than as a sheet. Green algae never looks like that.

One more distinction worth making: not everything dark is black algae. Cobalt spotting on some plaster and a fine dark mottling from metals both get misdiagnosed constantly, and both get treated with weeks of pointless brushing.

Why does green algae clear so much faster?

Because green algae is suspended and exposed. It has no cap, no anchor and no defense beyond numbers. Hold an appropriate free chlorine level for your stabilizer, keep the filter running, and it is dead in a day or two and filtered out in a few more.

Green also announces itself early. The water goes dull, then hazy green, then opaque, and any of those stages is easy to reverse. It rewards a fast response, which is why a homeowner who tests after a storm rarely ever sees a serious green pool.

The mistake is applying the green playbook to black. Shocking a pool with black algae raises the chlorine, does very little to the colonies, burns money and leaves the owner convinced the product was bad. The chemical was fine. It could not get in.

How is black algae actually removed?

You break the cap. On a plaster or gunite pool, that means a stainless steel algae brush worked hard directly on each spot until the surface is opened up. On vinyl or fiberglass, use a stiff nylon brush instead, because steel will destroy those surfaces. The brushing is not preparation for the treatment. It is most of the treatment.

Then treat the opened spot immediately, while it is exposed. A common field method is holding a trichlor tablet against the spot, or rubbing the spot with one, so a high concentration of chlorine sits directly on the colony rather than being diluted into twenty thousand gallons. Some technicians use a chlorine paste for the same reason.

Then hold an elevated free chlorine level across the pool for several days, keep the pH at the low end of range so that chlorine is in its most active form, and repeat the brushing every single day. A specialist algaecide, usually a copper free polymer, can help hold the ground you win, but it does not replace the brush.

Then expect to do it again. Black algae comes back from a single surviving colony, so the routine continues for weeks after the spots visually disappear. Owners who stop at day four almost always see it again by the end of the month.

What lets black algae get established in the first place?

A rough or porous surface. Old plaster that has lost its smooth top layer, an acid washed surface, exposed aggregate that has worn open, or grout lines that have eroded. Black algae needs somewhere to hold, and a smooth surface gives it very little.

Low circulation and low chlorine in the same corner. Steps, the shady wall, behind a ladder rail, the dead spot where the return does not reach. Those are where colonies start, every time.

Contamination brought in from outside. Black algae often arrives on swimwear, floats or gear that was in a natural body of water. Lake Grapevine and Lake Lewisville both count.

So the prevention is unglamorous. Brush the shaded and rough areas weekly rather than the walls you can see from the patio, keep chlorine held rather than swinging, and rinse gear that came from a lake before it goes in the pool.