Algae in a pool is a chemistry failure — something in the water balance has allowed algae to establish faster than the sanitizer can kill it. The right treatment approach depends on the type of algae you're dealing with. Treating them all the same way produces inconsistent results. Here's how to identify and eliminate each type correctly.
Green algae (most common)
Green algae is the most common type and the most responsive to treatment. It turns the water green or teal and makes surfaces slippery. It's free-floating (suspended in the water and on surfaces) and responds well to chlorine shock when pH is correct and cyanuric acid is in range.
Why it happens: Free chlorine drops below effective levels, allowing algae spores (always present in the environment) to germinate and multiply. High phosphate levels are a major contributing factor — phosphates feed algae faster than chlorine can kill it, creating a situation where chemistry appears adequate but algae persists.
Treatment sequence:
- Test pH, CYA, and phosphates before adding anything. pH must be 7.2–7.4 for shock to be effective. CYA above 80 ppm may require a partial drain before treatment will work. Phosphates above 500 ppb should be treated before or alongside shock.
- Brush all pool surfaces thoroughly — disrupts the algae's protective biofilm and exposes cells to the chlorine you're about to add.
- Shock with calcium hypochlorite (not stabilized chlorine) — 2–4 lbs per 10,000 gallons depending on severity. Target 15–30 ppm free chlorine.
- Add a polyquat algaecide after shocking to prevent regrowth.
- Run filter continuously — clean/backwash every 8–12 hours as dead algae cells load the filter media.
- Vacuum dead algae to waste once the water begins clearing.
Timeline: Light green pools often clear in 24–48 hours. Moderate cases: 48–72 hours. Dark green or swamp-level blooms may require multiple shock treatments over 3–5 days.
Yellow / mustard algae
Yellow or mustard algae is a chlorine-resistant strain that clings to pool walls, steps, and surfaces — particularly in shaded areas. It looks like yellow-brown dust or sand that brushes off surfaces easily, only to reappear within days. It's harder to eliminate than green algae and requires a more aggressive approach.
Identifying characteristics: Yellow or mustard color, settles on surfaces and brushes off as powder, appears primarily in shaded corners and on steps. Returns quickly after brushing.
Why it's harder: Mustard algae has higher chlorine resistance than green algae and can survive at chlorine levels that prevent green algae from establishing. It's also capable of surviving on pool accessories, toys, swimwear, and cleaning equipment — reintroducing itself to the pool after treatment if these aren't addressed.
Treatment:
- Remove and treat all pool equipment, toys, and cleaning tools — sanitize them separately before returning to the pool.
- Brush all surfaces aggressively.
- Shock heavily — target 20–30 ppm free chlorine using cal-hypo.
- Use a mustard algae-specific algaecide — polyquat at double normal dosing, or a product specifically labeled for yellow/mustard algae.
- Vacuum to waste after brushing — don't recirculate dead algae through the filter.
- Run filter continuously and clean frequently.
- Maintain elevated chlorine (5+ ppm) for several days after treatment to prevent regrowth.
Timeline: Mustard algae typically requires 3–5 days of treatment and vigilant maintenance for 2 weeks afterward to confirm elimination.
Black algae
Black algae is the most stubborn type and requires the most aggressive treatment. It appears as dark blue-black spots on pool surfaces — most commonly in the grout lines of tile or in rough plaster surfaces where it can anchor deeply. Unlike green or mustard algae, black algae has a hard outer protective layer that's nearly impermeable to chlorine, and its root system penetrates into the pool surface.
Identifying characteristics: Dark spots (navy to black) that are firmly attached to the surface and don't brush off easily. Rough pool surfaces and grout lines are the most common locations. It's most common in plaster and pebble pools; rare in vinyl or fiberglass pools.
Why it's hardest to eliminate: The protective outer layer and deep root structure mean that chlorine can't penetrate to kill the full organism even at high concentrations. Treatment requires physical disruption of the outer layer before chemical treatment can be effective.
Treatment:
- Use a wire brush (for plaster surfaces) to aggressively scrub each black algae spot — you need to break through the protective outer layer. This is physically demanding but essential.
- Apply a trichlor tablet or granular cal-hypo directly to each scrubbed spot while the pump is off — direct contact application gives the chlorine its best chance at penetrating the organism.
- Shock the pool heavily — 3–4 lbs of cal-hypo per 10,000 gallons, targeting 20+ ppm.
- Add a copper-based algaecide — copper is specifically effective against black algae's protective layer. Note: copper algaecides can stain light-colored pool surfaces at high doses. Follow dosing instructions carefully.
- Repeat brushing, direct application, and shocking every 2–3 days.
Realistic expectations: Black algae is very difficult to fully eliminate without professional help and sometimes without acid washing or replastering. If spots continue to return despite aggressive treatment, professional assessment is the appropriate next step. Black algae that has penetrated deeply into plaster may require resurfacing to fully remove.
The phosphate connection
Regardless of algae type, elevated phosphate levels significantly increase treatment difficulty and the likelihood of recurrence. Phosphates are the primary nutrient source for algae — high phosphate levels allow algae to establish faster than chlorine can address it. If you're treating algae and it keeps returning despite adequate chemistry management, test phosphates. If they're above 200–500 ppb, treat with a phosphate remover as part of (or before) your algae treatment protocol. This is one of the most commonly missed steps in algae remediation.