Pool algae: identify and kill it
Pool algae comes in four common forms: green, mustard (yellow), black, and pink slime, which is actually a bacterium.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (9 sections)
All of them are treated the same basic way: brush hard, clean the filter, and use a chlorine product labeled for algae control at the amount its label gives for your pool. Then keep brushing and testing until chlorine stops dropping overnight.
Identify by color and feel
Look where it grows and rub it with your fingers or a brush. Texture tells you as much as color.
| Type | What you see | How it feels and brushes | Where it grows |
|---|---|---|---|
| Green | green tint in the water, green film on walls | slimy; brushes off and clouds the water | anywhere, starting in corners and steps |
| Mustard (yellow) | yellow-brown dust or patches | brushes off easily, then settles back | shady walls and floor |
| Black | dark blue-black spots, pinhead to coin size | raised, hard to brush off | plaster and grout, rough surfaces |
| Pink slime | pink or red slimy film | slick, wipes off | skimmer, returns, light niches, PVC |
| White water mold | white or gray clumps like wet tissue | soft, falls apart | pipes, filter, returns |
Not sure it's algae? Dirt settles in the same places as mustard algae but doesn't come back overnight. Metal stains don't brush off at all and often lighten when you rub a vitamin C tablet on them; see metal stains.
Green
Green algae is the most common and the easiest to kill. It grows fast when chlorine drops below the minimum for your CYA and can turn a clear pool green within days in warm weather. Treatment: balance pH, brush, run the filter continuously, and add a chlorine product labeled for algae control at the amount its label gives for your gallons. Follow the label for any repeat dose, and keep brushing until the water is clear and chlorine holds overnight. The calculator above shows the everyday chlorine minimum for your stabilizer (CYA); for the treatment dose, it points you to your label. If the whole pool is green, follow the full green water recovery plan.
Mustard (yellow)
Mustard algae is more tolerant of chlorine than green algae and likes shade. It brushes away easily, drifts as a cloud, settles again, and seems gone until it comes back the next morning. Spring pollen can look the same; the pollen vs algae check tells them apart. The full treatment, including what to do with toys and swimsuits, is on the mustard algae page.
Black
"Black algae" is not algae. It is a cyanobacterium that forms raised colonies with a protective outer layer and anchors into porous plaster, so chlorine alone struggles to reach it. Spots that don't brush off at all are more likely stains. Brushing, brush type by surface, and how long treatment takes are on the black algae page.
Pink / white water mold
Pink slime is a bacterium (reported to be in the genus Methylobacterium), not an alga. White water mold is a fungus-like growth. Both prefer low-flow, shady spots inside the equipment: skimmer walls, return fittings, light niches, the pump basket, and inside pipes. Because they live where pool chlorine circulates poorly, a shock alone often misses them: scrub the fittings and clean the filter too. Step-by-step cleanup: pink slime in a pool.
Algaecides: when they help
Algaecides are a supplement to chlorine, not a substitute. Three main kinds are sold:
- Copper-based products can be effective, but copper can stain plaster and tint blond hair green. Watch your metal levels if you use them.
- Quaternary ammonium ("quat") products are surfactants and can foam, especially when overdosed. See foamy water.
- Polyquat products are generally sold as non-foaming and non-staining. They tend to cost more.
Where an algaecide makes sense: as a weekly preventive in pools that are hard to keep at the right chlorine level, or for a stubborn spot you have already brushed and shocked. Where it doesn't: as a replacement for a shock when chlorine is low. Dose exactly by the label, use only an algaecide registered for pools, and never mix it with chlorine in the same bucket. More is not better.
Prevention: keep FC above your CYA minimum
Algae usually takes hold when chlorine drops below the minimum for your stabilizer. Keep FC at or above these levels every day, not just after you add chlorine, and within the swim limit on your label:
| CYA | Liquid chlorine, bleach, or tablets | Salt water generator (usually run at CYA 60–80) |
|---|---|---|
| 0 | 1 | — |
| 30 | 2.5 | — |
| 40 | 3 | — |
| 50 | 4 | — |
| 60 | 4.5 | 3 |
| 70 | 5.5 | 3.5 |
| 80 | 6 | 4 |
Minimum = 7.5% of CYA (5% for salt cells), at least 2 ppm with stabilizer, rounded up to 0.5 ppm. This is our own model; see the methodology. Pool product labels often say not to swim above 4 ppm. Where the minimum for your CYA is above your label's limit (CYA 60 and up in this table), follow the label and lower CYA by replacing some water; keeping CYA at 30–50 lets both fit. Salt cell values follow the cell maker's guidance.
Other habits that help: brush weekly, keep the filter clean and the pump running long enough, and test more often in hot weather and after heavy use. Phosphate removers reduce one algae nutrient; they don't kill existing algae, and chlorine at the right level matters more (see phosphates in pools). If chlorine keeps vanishing even though you add enough, look for hidden algae with the overnight loss test on this page about chlorine loss. For everyday targets, see pool chlorine levels.
Safety. Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling. Never mix chlorine with algaecide, acid, or another chlorine product; add each one separately to the water with the pump running. Wear gloves and eye protection. No swimming until the water is clear and chlorine is back at or below the limit on your label. Poison Control: 1-800-222-1222.
FAQ
Can you swim in a pool with algae?
No. Visible algae means chlorine has been below the level needed to control germs. Algae can also hide swimmers from view and make surfaces slippery. During treatment, chlorine can be above the swim limit on the label, so wait until the water is clear and free chlorine is back at or below your label's limit.
What causes pool algae?
Free chlorine falling below the minimum for your CYA, usually after hot weather, heavy use, rain, missed doses, or a stabilizer level that has crept up. Spores arrive constantly with wind, rain, and swimmers; they only grow when chlorine gives them room.
Does pool algae eat chlorine?
Algae doesn't eat it, but chlorine is used up as it oxidizes algae. That is why a pool with algae loses chlorine quickly, and why a drop of more than 1 ppm overnight is a classic sign of hidden algae.
Sources
- Orenda, Understanding algae (types, black algae as cyanobacteria, white water mold): blog.orendatech.com (a chemical maker's blog)
- Wikipedia, Pink algae (Methylobacterium): wikipedia.org
- The Conversation, why pool water can turn hair green (copper): theconversation.com
- CDC, Model Aquatic Health Code 2024, Annex (chlorine and CYA): cdc.gov (PDF)
- Taylor Technologies, K-1005 instruction card, trouble prevention chart (brush type by surface): PDF
- Hayward, AquaRite manual (salt cell CYA): PDF