Dead algae looks like fine gray, white, or light-brown dust that sinks and settles on the pool floor (live algae is green, yellow, or black, feels slimy, and clings to walls). Brushing dead algae lifts it into a cloud that resettles within hours. To remove it, vacuum to waste or vacuum through the filter and then clean the filter; a clarifier or flocculant helps the fine particles drop. If green, brown, or gray stains remain on the surface after the pool is clear, that is staining — not algae — and it usually takes an acid or chlorine wash to remove.
Dead algae vs. live algae: how to tell.
Algae changes color and behavior when it dies. Live algae is alive and holding on; dead algae has let go and is just sitting there. A quick side-by-side:
| Dead algae | Live algae | |
|---|---|---|
| Color | Gray, white, light tan, or dull brown | Green (most common), mustard-yellow, or black |
| Where it is | Settled on the floor, in corners, on steps and the shallow end | Clinging to walls, steps, behind ladders; green water when it's bad |
| Sink or float? | Sinks. It's dead weight — it drops once the water goes still | Floats or clings. Green algae can cloud the whole water column |
| Brush test | Lifts into a puffy cloud, then resettles in the same spot | Smears or comes off in slimy sheets; feels slippery |
| After you shock | This is what's left behind | Turns gray and dies within 24 hours if chlorine is high enough |
Why dead algae won't just filter out.
Dead algae particles are tiny — much smaller than the dirt and leaves your filter is built to catch. Sand and cartridge filters let a lot of it pass straight back into the pool, which is why it seems to "come back" every day after you brush. It isn't growing. It's the same dust, recirculated.
How to remove dead algae, step by step.
- Confirm the algae is actually dead. Test the water. Free chlorine should be high (shock level) and the water should have turned from green to cloudy gray-blue. If it's still green, keep chlorine up and brush daily until it turns.
- Let the water go still. Turn the pump off for several hours or overnight so the fine particles settle on the floor.
- Vacuum slowly to waste. If your filter has a multiport valve, set it to Waste so the dead algae leaves the pool instead of passing through the filter. Move slowly — fast strokes stir it back up. (You'll lose some water; top off afterward.)
- No waste setting? Vacuum through the filter and clean it after. Cartridge filters: hose off the pleats. Sand or DE filters: backwash. A dirty filter after vacuuming is normal — that's the algae you just removed.
- Use a clarifier or flocculant for the last of it. A clarifier clumps fine particles so the filter can catch them. A flocculant drops everything to the floor so you can vacuum it to waste the next morning. Follow the label — floc is a one-shot product.
- Rebalance the water. Shocking knocks pH and alkalinity around. Bring them back into range so the surface doesn't stain or scale while it recovers.
- Run the pump long enough. In an Arizona summer that's typically 8–12 hours a day. Short run times are how pools get algae in the first place.
Arizona tip: Our algae blooms usually follow the same script — a monsoon dust storm, a week of 110° heat, and a pump that's only running four hours a day. If your pool went green after a storm, assume the filter is loaded with dust too and clean it as part of the cleanup.
When "dead algae" is actually a stain.
If the water is crystal clear, the vacuum pulls up nothing, and you still see green, brown, or gray patches on the floor or steps — that isn't algae anymore. It's staining left behind in the surface. Algae that sat for weeks, metals that dropped out when you shocked, and the Valley's hard-water minerals all leave marks that no amount of brushing will lift.
A stain test is simple: hold a chlorine tablet against the spot for a few minutes. If it lightens, it's organic (algae) staining and a chlorine wash will take care of it. If it doesn't budge, it's mineral or metal staining and an acid wash is usually the fix. Both are done with the pool drained, and we always clean the waterline tile in the same visit since the pool is already empty.
Black and mustard algae are different animals.
- Mustard (yellow) algae looks like pollen or fine sand on the shaded side of the pool and brushes off easily — then returns. It's chlorine-resistant, so standard shock often isn't enough. It needs a higher shock dose and everything that touched the water (brushes, floats, swimsuits) cleaned.
- Black algae shows up as dark spots that feel like raised dots and won't brush off. It roots into porous surfaces. Scrubbing the spots open with a steel brush before shocking helps, and on a badly rooted surface the honest answer is sometimes a chem wash or a new pebble finish.
How to keep it from coming back.
- Keep free chlorine in range year-round — Arizona pools don't get a winter break from algae, just a slower one.
- Run the pump 8–12 hours a day in summer, more after a dust storm.
- Brush walls and steps weekly. Algae starts where the water doesn't move.
- Clean the filter on a schedule, not just when the pressure gauge screams.
- If the surface is rough and pitted, algae has a thousand places to hide. A worn finish is one of the most common reasons an East Valley pool "always goes green."
