How to Identify, Remove, and Prevent Algae in Your Pool.
Algae in a pool is a visible bloom that appears when chlorine is too low, pH is out of range, or circulation is weak. Spores enter through wind, rain, swimwear, and equipment — but balanced water keeps them under control. The four types — green, mustard, black, and red — each respond differently to treatment, so identifying the type first is what lets you shock, brush, and vacuum correctly and stop it from coming back.
Pool Algae Types: How to Tell Them Apart
Four types of algae appear in residential pools: green, mustard, black, and red (also called pink slime). The table below is the fastest way to identify what you are looking at before choosing a treatment path. Each type is described in more detail underneath, with a link to its full treatment guide.
| Type | How to identify | Where it appears | Brushes off? |
|---|---|---|---|
| Green | Green water, or a slippery green film on walls and floor | Free-floating, or on any wall, floor, or step | Yes, easily |
| Mustard | Yellow or brown sandy, powdery 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, and cleaner baskets | Yes, but lives inside equipment |
Green Algae
The most common and easiest type to clear, responding to a correctly dosed shock at the right pH within 48–72 hours. If a green pool has not cleared after 72 hours with a correct-dose shock, cyanuric acid above 80 ppm is almost always the reason.
Full green guideMustard Algae
Chlorine-resistant strain mistaken for pollen, dirt, or fine sand. It brushes off but reattaches within hours unless every brush, net, vacuum head, and cleaner is disinfected 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 of these at once — which is why testing only chlorine rarely tells the full story.
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 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 shows up anywhere else.
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 in a Pool
Killing algae is a six-step sequence done in order: correct pH, brush, shock at a dose that matches the algae type, add an algaecide if the type requires it, vacuum the dead material out, and clean the filter. The six steps are the same for every algae type — only the shock dose and a few type-specific details change. 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 around 20%. If pH is above 7.6, add muriatic acid or sodium bisulfate, run the pump for 30 minutes, and retest before continuing.
Brush every surface
Brush walls, floor, steps, corners, and the waterline before shocking. For green and mustard algae, brushing breaks up surface colonies and drives them into the water where chlorine can reach them. For black algae, a stainless steel wire brush is needed to scrub each spot and break the protective outer layer — without this step, 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 timing vary by type:
| Algae Type | Shock Dose (per 10k gal) | Key Step | Clears In |
|---|---|---|---|
| Green (moderate) | 2–3 lbs cal-hypo | Brush all surfaces first | 48–72 hours |
| Green (severe) | 4–6 lbs cal-hypo | Brush all surfaces first | 48–72 hours |
| Mustard | 3 lbs cal-hypo | Clean every brush, net, and cleaner before brushing | 3–5 days |
| Black | 3 lbs cal-hypo | Wire-brush each spot to bare plaster | 5–10 days |
| Red / Pink | Raise FC to 5–10 ppm | Clean the interior of all equipment and hoses | 24–48 hours |
The table gives the parameters, but the failure point behind each type decides whether the treatment holds. Green algae that has not cleared in 72 hours is almost never a dose problem — retest cyanuric acid first. Mustard algae returns reliably when one contaminated brush or cleaner goes back in, so the equipment-cleaning step is not optional. Black algae spots that look faded but were not scrubbed to bare plaster still hold viable roots and regrow within days. For pink slime, any piece of equipment whose interior cannot be fully cleaned should be replaced, since the biofilm reestablishes from surviving interior surfaces.
Add algaecide when needed
Once free chlorine drops to around 5 ppm after shocking, add an algaecide matched to the type. Mustard algae responds to a quaternary (quat) or copper algaecide, and black algae needs a copper-based product rated for it. Green and red algae do not require algaecide if the shock is correctly dosed.
Adding algaecide while chlorine is still very high degrades the product before it can work, and a foaming quat algaecide should not be used in pools with a spa or water feature. For how copper-based products work and when to use them, does copper kill algae goes into detail.
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 — since fine dead particles pass through most filter media and return through the jets within hours. A vacuum cannot kill live algae or pull it off walls, so vacuuming without shocking first only moves live cells around. For cartridge setups without a waste option, vacuum on filter mode and clean the cartridge immediately.
How to remove algae without a vacuum covers the clarifier-and-filter method when no vacuum is available, and does pool vacuum remove algae explains what a vacuum can and cannot do.
Clean and backwash the filter
A filter loaded with dead algae is one of the most common causes of rapid regrowth, because even a well-shocked pool gets reseeded if the filter is left untouched. Backwash sand or DE filters, and soak cartridges in filter cleaning solution for at least an hour to remove embedded algae. Run the pump another 24 hours and retest before clearing the pool for swimmers. The pool is ready when free chlorine is back to 1–3 ppm, pH is 7.4–7.6, and the water is visually clear.
How to Prevent Algae From Coming Back
Preventing algae comes down to steady chemistry, regular brushing of dead zones, and enough pump runtime. A pool where chlorine stays above 1 ppm consistently rarely develops a bloom.
Maintain chlorine & pH
Keep free chlorine at 1–3 ppm and pH at 7.4–7.6 through swimming season. Test 2–3 times a week.
Shock weekly
A weekly maintenance shock of about 1 lb cal-hypo per 10,000 gallons keeps chlorine demand from building. High bather load or heat may call for twice-weekly shocking.
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 and refilling.
Lower CYA →Brush dead zones
Brush corners, steps, tanning ledges, and behind ladders at the start of each weekly cleaning. Run the pump 8–12 hours a day to turn over the full pool volume.