Pool algae types and how to get rid of each one
Every type of pool algae comes back to the same root cause: free chlorine too low for long enough, relative to the stabilizer level. What differs is where it grows and how stubborn it is. Here's how to tell them apart, and what each needs beyond the usual shock.
Quick identification
| What you see | Likely cause | Difficulty |
|---|---|---|
| Green water, or green film on walls and floor | Green algae | Easiest to clear |
| Yellow-brown or mustard-colored powder on shady walls, which brushes off easily and comes back | Yellow (mustard) algae | Moderate: it hides on equipment and toys |
| Dark blue-green or black raised spots in plaster or grout, which don't brush off | Black algae (a type of cyanobacteria) | Hardest: it roots into the surface |
| Pink or reddish slime in corners, fittings or the skimmer; white, tissue-like clumps in water or pipes | Pink slime or white water mold (bacteria and fungi, not algae) | Moderate: it hides in plumbing and crevices |
| Brown or green stains that don't feel slimy and don't brush away | Probably a stain, not algae | See pool stains |
A quick check: rub the spot with a chlorine tablet in a sock, or brush it hard. Algae usually loosens or lightens. Metal stains don't respond to chlorine.
The treatment every type needs
Whatever the type, the core fix is the same: raise free chlorine to the shock level for your stabilizer, and hold it there until the algae is gone.
- Test free chlorine, pH and CYA. Lower pH to about 7.2–7.4.
- Use the shock calculator to find the shock level for your CYA (a common guideline is about 40% of CYA) and the dose.
- Brush the whole pool, run the pump continuously, and clean the filter as the pressure rises.
- Retest and top up chlorine several times a day to hold the shock level.
- Stop only when the water is clear, combined chlorine is 0.5 ppm or less, and the pool loses no more than about 1 ppm of free chlorine overnight.
The full process is in how to shock a pool. If CYA is very high, lower it by partly draining first, or the shock level becomes impractical.
Green algae
The most common type. It floats in the water and coats surfaces, and can turn a pool from slightly cloudy to green in a day or two in warm weather. It responds well to a proper shock. Most "it came back" stories come from stopping too early, when the water turned from green to cloudy blue.
Yellow (mustard) algae
It grows in shady spots and on walls out of direct sun. It brushes off easily but returns within days, because it survives on things outside the water's main flow. Beyond the standard shock:
- Hold the shock level for longer than for green algae, and brush daily.
- Clean everything that was in the water: toys, floats, brushes, pool cleaners, ladders and steps. Wash swimsuits too.
- Clean the filter thoroughly at the end, or replace the media if it's heavily contaminated.
Black algae
Black algae forms dark spots with a protective layer and roots that reach into plaster and grout. That's why it survives normal chlorine levels and returns in the same places. It mostly affects plaster, pebble and other cement finishes.
- Break the surface layer. On plaster, scrub each spot hard with a stainless steel brush. Don't use steel brushes on vinyl, fiberglass or painted surfaces, which they will damage.
- Apply chlorine directly. On plaster, some owners rub the spots with a trichlor tablet after brushing. Never do this on vinyl or fiberglass.
- Shock and hold for longer than for other types, brushing the spots daily, until they stop returning.
Expect black algae to take persistence: missing a few days can let it regrow from the roots.
Pink slime and white water mold
These aren't algae. They're bacteria and fungi that form a protective slime and often live inside pipes, skimmers, light niches, ladder bumpers and the filter. Clearing them means shocking and holding the shock level while brushing everywhere you can reach, including inside the skimmer and around fittings. Clean or replace filter cartridges at the end. White water mold in particular may need the plumbing cleaned and the shock held for several days.
Preventing algae
- Keep free chlorine at or above about 7.5% of your CYA every day (about 5% with a salt cell). The chlorine and stabilizer guide has a table.
- Don't let CYA creep up unchecked from tablets or dichlor.
- Run the pump long enough each day, and keep the filter clean.
- Brush weekly, especially shady corners and steps.
- Phosphate removers and algaecides are sometimes used as extras. They can help in some cases, but they don't replace keeping chlorine at the right level.
Frequently asked questions
Is it safe to swim with algae in the pool?
No. Algae itself is not usually the main danger, but it shows chlorine has been too low to control germs, and cloudy water hides swimmers in trouble. Wait until the water is clear and chlorine is back in range.
Can I just use algaecide?
Algaecide alone rarely clears an established bloom. Shocking and holding chlorine does the work. Some algaecides contain copper, which can stain plaster, so read the label.
Why does algae come back after a week?
Usually because the shock was stopped before the overnight chlorine loss test passed, CYA is high so the normal chlorine level is too low, or algae survived on equipment and toys.