Pool Algae: Types, Prevention & Treatment

The three common types

Type	What it looks like	Where it appears	Difficulty
Green	Green water or a slippery green coating	Everywhere; starts in corners and on steps	The easiest
Yellow	Yellowish or brownish powder that looks like sand or pollen	Shaded walls, the floor	Moderate, comes back easily
Black	Dark spots with a raised texture	Seams, around inlets and steps	The most resistant

You can tell yellow algae from dust with a simple test: brush it. If it lifts easily and reappears in the same spot within one or two days, it is algae. "Black algae" is actually a colony of cyanobacteria, protected by layers that make it hard for chlorine to get through.

Treatment without damaging the liner

A liner needs gentle handling: a brush with nylon bristles, never a metal or wire brush, and chemicals always dissolved in the water, never in direct contact with the lining.

Green algae

Brushing, pH adjustment and shock chlorination. If the water has already turned green, follow the steps for green pool water.

Yellow algae

Brush the affected spots every day, keep chlorine high for several days with repeated shock chlorination and clean the filter. Because it clings to objects, disinfect everything that goes into the water: ladder, toys, brushes, vacuum, even swimsuits.

Black algae

This takes patience. Brush hard, always with a nylon brush, to break the protective layer, and shock straight afterwards. Repeat for several days. An algaecide suitable for black algae helps.

Popular tips such as rubbing the black spot with a chlorine tablet or leaving the tablet on it are not suitable for a liner pool: they bleach and damage the PVC within a short time.

Pink slime and white water mould

A pink or white slimy coating inside the skimmer, behind the ladder or around the inlets is not algae but bacteria hiding in places chlorine struggles to reach. It is treated the same way: brushing, shock and cleaning all the hidden spots, including the baskets and the inside of the skimmer.

In every case, once treatment is complete, give the filter a thorough clean or clean the cartridge, so no residue is left inside to recontaminate the water.

Algaecides: when they help

Algaecides do not replace chlorine; they complement it. A small preventive dose each week helps in pools with shaded corners or recurring problems. With a liner, especially a light-coloured one, choose copper-free algaecides, because metals can leave stains, as we also explain for ionization. Some algaecides foam at high doses, so stick to the dosage.

Prevention

Free chlorine at 1–3 mg/l without gaps, and pH at 7.2–7.6.
Stabilizer no higher than recommended, because too much of it slows chlorine down; see chlorine stabilizer.
Good circulation: return inlets angled to move water into the corners and behind the steps, where algae usually start. More in pool water circulation.
Brush the walls and steps once a week.
Use a cover at night and when it is windy, to keep leaves and dust out.

Questions

Can I swim when there is a little algae?

Better not until the chlorination is complete. Algae show that chlorine is not sufficient, so bacteria may survive too, and slippery steps increase the risk of a fall.

Why do algae keep coming back in the same spot?

Usually because the water barely moves there or the spot is shaded, or because algae remained on fittings and tools. Point the return inlets towards the spot, brush it more often and disinfect your cleaning tools.

Green, yellow, black: how to recognize and stop algae