Algae in the Pool — Identify, Treat, and Prevent.
Pool algae is a visible bloom that appears when chlorine is too low, pH is out of range, or circulation is weak. Green, mustard, black, and red algae each respond differently, so identifying the type first is the fastest path back to clear water.
How to tell the four pool algae types apart
Four types of algae appear in residential pools: green, mustard, black, and red (pink slime). The table below is the fastest way to identify what you are looking at before choosing a treatment path.
| Type | How to identify | Where it appears | Brushes off? |
|---|---|---|---|
| Green | Green water or slippery green film on walls and floor | Free-floating, or any wall, floor, or step surface | Yes, easily |
| Mustard | Yellow or brown sandy coating that looks like fine dust | Shaded walls, corners, and low-sun areas | Yes, but reattaches within hours |
| Black | Raised dark blue-green or black spots | Plaster, grout lines, concrete (rare on vinyl or fiberglass) | No |
| Red / Pink | Pink, orange, or reddish slime | Inside fittings, skimmers, hoses, robotic cleaner baskets | Yes, but lives inside equipment |
Green Algae
The most common and easiest to clear. Responds to a correctly dosed shock at the right pH in 48–72 hours. If it has not cleared after 72 hours, cyanuric acid above 80 ppm is almost always the reason.
Mustard Algae
Most often mistaken for pollen, dirt, or fine sand and vacuumed away without treatment. It reattaches within hours unless every brush, net, vac head, and robotic cleaner is sanitized before reuse.
Full mustard guideBlack Algae
The only type that does not brush off — each colony has a protective outer layer chlorine cannot penetrate. It rarely grows on vinyl or fiberglass, so raised dark spots on a liner are almost always something else.
Full black guideRed / Pink Slime
Actually a bacterium (Serratia marcescens), not a true alga. It lives inside equipment interiors as much as on open surfaces — which is why treating only the water lets it return within days.
Full pink guideWhy algae grows in a pool
Algae becomes visible when conditions let spores already in the water overwhelm the sanitizer. Four openings are responsible for almost every bloom, and most real-world blooms come from more than one at once.
Low free chlorine
Below 1 ppm, chlorine cannot suppress growth in warm weather. A pool can drop from 2 ppm to below 1 ppm within 24–48 hours of heavy sun and swimmer load.
High pH
At pH 7.8, only about 20% of the chlorine is in its active sanitizing form — so a 2 ppm reading can deliver less power than 0.4 ppm at correct pH.
Poor circulation
Corners, step alcoves, and tanning ledges get little water movement. Algae establishes in these dead zones before it becomes visible elsewhere.
High cyanuric acid
Above 80 ppm, CYA reduces chlorine effectiveness without changing the test reading. Partial draining is the only fix when levels are too high.
How to kill algae — a six-step sequence
Killing algae is a six-step sequence performed in order: correct pH, brush, shock at a dose that matches the type, add an algaecide if the type requires it, vacuum the dead material out, and clean the filter. Skipping or reversing any step is the main reason a pool clears briefly and turns green again within a week.
Correct pH to 7.2 – 7.4
Test pH before adding any shock. At pH 7.2–7.4, about 60–70% of chlorine is in its active hypochlorous acid form; at pH 7.8, that drops to roughly 20%. If pH is above 7.6, add muriatic acid or sodium bisulfate, run the pump 30 minutes, and retest.
Brush every surface
Brush walls, floor, steps, corners, and the waterline before shocking. For green and mustard algae, brushing drives colonies into the water column where chlorine can reach them. For black algae, use a stainless steel wire brush to break the protective outer layer — without this, shock cannot penetrate the colony at any dose.
Shock the pool by algae type
Shock in the evening with the pump running, after pre-dissolving the shock in a bucket of pool water and pouring it around the perimeter. Dose and sequence vary by type:
| Type | Dose (per 10k gal) | Before shock | After shock | Clears in |
|---|---|---|---|---|
| Green — moderate | 2–3 lbs cal-hypo | Brush all surfaces | Vacuum to waste after 12 hrs | 48–72 hrs |
| Green — severe | 4–6 lbs cal-hypo | Brush all surfaces | Vacuum to waste after 12 hrs | 48–72 hrs |
| Mustard | 3 lbs cal-hypo | Clean every brush, net, vac head, robot | Quat or copper algaecide at 5 ppm | 3–5 days |
| Black | 3 lbs cal-hypo | Wire-brush each spot to bare plaster | Trichlor on each spot daily 3–5 days | 5–10 days |
| Red / Pink | Raise FC to 5–10 ppm | Clean interior of all equipment, hoses | Reinspect equipment before restart | 24–48 hrs |
Green algae that has not cleared in 72 hours is almost never a dose problem — retest cyanuric acid. Mustard algae returns reliably when a single contaminated brush is reintroduced, so equipment cleaning is not optional. Black spots that look faded but were not scrubbed to bare plaster still contain viable root structures.
Add algaecide when needed
Once free chlorine drops to around 5 ppm after shocking, add an algaecide matched to the type. Mustard responds to quat or copper. Black algae needs a copper-based product rated for it. Green and pink don't require algaecide if shock is dosed correctly. Do not use a foaming quat in pools with a spa or water feature.
If algae returns in the same wall location after treatment, the surface likely has micro-cracks — direct algaecide application to that spot is more effective than re-treating the whole pool.
Vacuum dead algae to waste
Once the water shifts from green to grey or cloudy, dead algae has settled to the floor. Vacuum to waste — not through the filter. Fine dead particles pass through most filter media and return through the jets within hours. If a brown or yellowish layer reappears within 48 hours, it signals live mustard algae, not new growth.
Does a pool vacuum remove algae? → · Remove algae without a vacuum →
Clean and backwash the filter
A filter loaded with dead algae is one of the most common causes of rapid regrowth. Algae establishes inside cartridge filters where organic material accumulates in the pleats — a well-shocked pool will be reseeded if the filter is left untouched. Backwash sand or DE filters; soak cartridges in filter cleaning solution for at least an hour. The pool is ready when free chlorine is 1–3 ppm, pH is 7.4–7.6, and the water is visually clear.
How to prevent algae from coming back
Maintain chlorine & pH
Free chlorine 1–3 ppm, pH 7.4–7.6. Test 2–3 times per week. Algae rarely establishes in a pool where chlorine stays above 1 ppm consistently.
Shock weekly
1 lb cal-hypo per 10,000 gallons weekly keeps chlorine demand from accumulating. High-load or hot pools may need twice-weekly in peak season.
Manage cyanuric acid
Keep CYA between 30 and 50 ppm. Above 80 ppm, maintenance levels can't reliably prevent algae. The only correction is partial draining.
Brush dead zones
Brush corners, steps, and tanning ledges weekly. Run the pump 8–12 hours per day to turn over the full pool volume.