Common Pool Algae Types and How to Identify Them

Not all pool algae is the same. Green, yellow, black, and pink algae each have different causes, different treatments, and different prognoses. Identifying the type correctly the first time saves you days of chasing a green pool with the wrong chemicals.

Green algae

The most common pool algae. Free-floating in the water, gives the water a green tint, and forms slimy patches on walls and the pool floor. Reproduces extremely fast in warm, sunny, low-chlorine conditions.

Cause: low free chlorine, poor circulation, or high stabilizer (cyanuric acid) suppressing chlorine effectiveness.

Treatment:

1. Brush the pool walls and floor thoroughly.
2. Test and adjust pH to 7.4–7.6 (chlorine works poorly outside this range).
3. Triple-shock with cal-hypo or dichlor (10–15 ppm free chlorine).
4. Run pump 24/7 until water clears (usually 2–4 days).
5. Clean or backwash the filter daily during recovery.

Yellow / mustard algae

Sticks to shaded walls and corners. Looks like sand or pollen at first. Resistant to normal chlorine levels — you can have 3 ppm free chlorine and still grow yellow algae.

Cause: spores carried in from contaminated swimsuits, brushes, toys, or pets that swam in untreated water. Once introduced, it’s persistent.

Treatment:

1. Brush hard — yellow algae needs mechanical disruption.
2. Sanitize every pool toy and brush separately (don’t reintroduce spores).
3. Shock to 15–20 ppm free chlorine and hold for 48 hours.
4. Add a copper-based or polyquat algaecide labeled for mustard algae.
5. Brush daily for a week after water clears.

Black algae

The worst kind. Forms dark green-to-black spots that anchor into plaster and grout with roots. Resistant to normal chlorine and even most shock treatments. Once established, can take weeks to fully eliminate.

Cause: rough surfaces (plaster, gunite, grout) plus chronically low sanitizer levels. Once it gets into pool grout, it’s a years-long battle.

Treatment:

1. Mechanically scrub each spot with a stainless-steel brush to break the protective layer.
2. Apply granular cal-hypo directly to each spot (under water).
3. Triple-shock the whole pool to 20+ ppm.
4. Use a black-algae-specific algaecide (usually copper sulfate based).
5. Brush each spot daily for 2 weeks even after they look gone.

Pink slime (not actually algae)

Pink to dark pink film on walls, ladders, and around fittings. Technically a bacteria (Methylobacterium), not algae. Likes shaded, low-flow areas.

Cause: low sanitizer plus high phosphate levels.

Treatment: shock to 15 ppm, brush vigorously, add a phosphate remover, run pump 24/7 for 48 hours. Pink slime usually clears in a single shock cycle.

Chemicals you’ll need for any algae fight

Prevention beats treatment, every time

• Keep free chlorine at 2–3 ppm continuously.
• Maintain pH between 7.4 and 7.6.
• Test stabilizer monthly; keep at 30–50 ppm (60–80 for salt pools). Above 80 ppm, chlorine becomes sluggish.
• Brush the pool weekly even when you don’t see algae.
• Sanitize swimsuits, brushes, and toys that come from other pools or natural water bodies.

If you’re fighting an algae bloom that won’t clear with normal shock, send PST Pool Supplies a photo and a description of where the algae is concentrated. Black or mustard algae need different products than green — we’ll match the right chemicals on the first order.