How to Identify Pool Algae

The two tests

You do not need a microscope. You need to answer two questions, and the answers narrow it to one of three.

1. **Where is it?** Suspended through the whole water body, or attached to a surface? Green algae clouds the water; mustard and black attach.
2. **Does it brush off?** Take a brush to it. Green and mustard lift with almost no pressure. Black resists, and when it does come away it leaves a visible pit or stain in the plaster where it was anchored.

The three types, side by side

Green algae

The common one. It reproduces in the water column rather than on surfaces, which is why the whole pool changes color rather than developing patches. It is also the easiest to kill: a correctly sized shock breaks it in a day, and the remaining work is filtration.

Green algae in a pool with good chlorine is a sign that the chlorine is not doing what the number says. That usually means cyanuric acid, occasionally pH above 7.8. Go straight to clearing it, then find out why.

Mustard algae, and why it keeps coming back

Mustard algae is the one people mistake for dirt or pollen, because it is powdery, it sits on the floor and shaded walls, and it puffs into a cloud when disturbed. It brushes away completely - and then it is back in the same place the next day, which is the diagnostic.

It survives chlorine levels that kill green algae, and it survives on things that touch the pool: floats, brushes, swimsuits, the vacuum hose, pool toys. That is the real reason it recurs. Treating the water without treating the equipment gets you a two-week cycle.

• Shock to a higher level than you would for green algae and hold it for longer
• Brush the affected areas daily during treatment, not once
• Put every piece of pool equipment and every swimsuit through the shocked water, or wash them separately
• Run the filter continuously and clean it afterwards
• Use an algaecide the maker names for mustard specifically, rather than a generic one

Black algae, which is not really algae

Black algae is a cyanobacterium. It anchors into rough plaster and grout with root-like structures, and it grows a protective layer over itself that chlorine cannot penetrate at any reasonable concentration. That is the whole problem: the treatment is not chemical strength, it is breaking the layer.

It is far more common in gunite and plaster pools than in vinyl, because it needs a rough surface to anchor into. If you have a vinyl liner and you think you have black algae, it is much more likely to be a stain or mold on the liner surface.

1. Brush each spot hard with a stainless brush - the goal is to break the crust, not remove the spot
2. Apply a copper-based algaecide directly to the brushed areas, following the label
3. Maintain elevated chlorine while you work through it
4. Repeat every two or three days. Black algae takes weeks, not days
5. Check that pH stays in range throughout, because copper stains at high pH

Things that look like algae and are not

• **Green water with no growth on any surface** is often dissolved metal, not algae. Copper and iron do this, and shocking makes it worse.
• **Pink or orange slime** in corners and behind fittings is a bacterium rather than algae. It needs the same shock treatment plus scrubbing, but algaecide will not touch it.
• **A yellow-brown film after a storm** is often pollen or fine silt. Wipe some onto a white cloth: pollen smears yellow and rinses out, mustard algae does not.
• **White or gray flakes** are usually calcium, particularly if your water is hard and pH has drifted up. That is a balance problem, not a living one.