Dead algae but pool still cloudy?
If your pool is still cloudy after killing the algae, dead algae particles are suspended in the water — most pools clear in 24–72 hours once you switch from killing to filtering. Run the pump 24/7, clean or backwash the filter every 8–12 hours, and add a pool clarifier to speed things up. Don't stop shocking until you pass the overnight chlorine loss test. If you're not sure algae was the cause, see all causes of cloudy pool water first.
Why dead algae makes water cloudy
Algae cells are microscopic, but during a bloom there are billions of them. When chlorine kills them, the cells rupture and release their contents as fine particles that float in the water column. Those particles are too small for the human eye to see individually, but together they look like grey-white milk — and they're too small for most filters to catch in a single pass.
The cloudiness you're seeing is actually a sign the shock worked. The bad news: it can take longer to clear the dead algae than it took to kill it.
First — confirm the algae is actually dead
Before you spend three days trying to clear "dead algae," make sure you're not still feeding a live bloom. Run the Overnight Chlorine Loss Test (OCLT):
- At sunset, test and record FC.
- Don't add any chlorine overnight. Pump runs as normal.
- At sunrise (before the sun hits the pool), test FC again.
- If FC dropped by 1 ppm or less → algae is dead. Proceed with clearing.
- If FC dropped more than 1 ppm → algae is still alive and consuming chlorine. Keep holding shock level.
The OCLT works because UV is the main daytime chlorine consumer, and at night nothing should burn it except organics. A pool with no algae and no organics loses essentially zero FC overnight.
Stop here if OCLT fails
If your FC drops more than 1 ppm overnight, the algae isn't dead yet. Hold FC at shock level until OCLT passes — usually another 1–3 days. See the full shock-and-hold process.
The clearing process
Once OCLT passes and you confirm the algae is dead, switch gears from killing to clearing. The chemistry is settled — now it's a filtration problem.
- Stop holding shock level. You can let FC drift back down toward your normal CYA-based target. No more daily chlorine top-ups.
- Keep the pump running 24/7. Every hour off lets particles re-settle. Filtration is the entire game now.
- Brush daily. Re-suspend any settled particles so the filter can grab them.
- Clean the filter every 8–12 hours. A loaded filter does nothing. Backwash sand/DE, hose down cartridges. Expect the cleaning to be heavy with dead-algae sludge.
- Test FC and pH daily. The water should hold FC well now — that's the signal everything is on track.
Pool clarifier vs flocculant — when to use each
Both products help the filter catch the fine dead-algae particles. They work differently and you should choose carefully.
| Clarifier | Flocculant (floc) | |
|---|---|---|
| How it works | Bonds particles into larger clumps the filter catches | Drops all particles to the floor as a sludge layer |
| Speed | 24–48 hours | Overnight (8–12 hours) |
| What to do next | Let filter do the work | Vacuum to waste (NOT through the filter) |
| Water loss | None | Significant — replace what you vacuum out |
| Best for | Light-to-moderate haze, no time pressure | Heavy cloudiness, willing to lose water for speed |
When to use clarifier
Clarifier is gentle and forgiving. Add the dose on the label, wait, run the pump. It works best when chemistry is balanced and you just need a nudge to finish the job. Most pool owners only need clarifier — not floc.
When to use flocculant
Use floc as a last resort when cloudiness has lingered for days and you can't wait. Floc drops everything to the floor overnight as a fine sludge — you then have to vacuum to waste, which dumps water through the waste line instead of back through the filter. This loses 6–12 inches of water but clears the pool fast.
Critical: never run floc through your filter. It clogs sand and ruins DE/cartridge media.
Filter-type tips for clearing dead algae
| Filter type | Clearing tip |
|---|---|
| Cartridge | Pull and hose cartridges every 8–12 hours. Have a spare set; rotate while one is drying. Soak in cleaner overnight after clearing. |
| Sand | Backwash every 6–8 hours. Add DE powder through the skimmer (about 4 oz per 10 sq ft) for finer particle capture during clearing only. |
| DE | Backwash and recharge with fresh DE. DE filters clear dead algae fastest — they catch the smallest particles. |
How to know you're done
Declare victory when ALL of these are true:
- Water is clear enough to see the bottom from any angle
- Combined chlorine (CC) is 0.5 ppm or below
- OCLT passes (FC drops 1 ppm or less overnight)
- FC is holding at your normal CYA-based target without daily intervention
If the water is clear but CC is still above 0.5, there are still organics being broken down — keep filtering. If OCLT fails again, you didn't fully kill the algae; raise FC back to shock level and hold it until OCLT passes.
What if the water won't clear?
If you've been at it for more than 5–7 days and the pool is still cloudy:
- Check CYA. If above 80 ppm, your effective chlorine is too low to finish the kill. Partial drain and refill.
- Check the filter. A failing filter (broken laterals, channeling sand, torn cartridges, missing DE) won't catch particles regardless of run time.
- Test calcium hardness. CH above 500 ppm can cause persistent cloudiness even with clean filtration. Partial drain only.
- Try floc as last resort — when all else fails, this works but costs you water.
Catch problems before a shock is needed
PoolChem Tracker tracks every reading and warns you when FC is below your CYA minimum — the warning that prevents the next algae bloom.
Cloudy & Green Pool Series
Related reading
- FC/CYA Chart — the chlorine target that prevents algae returning
- Why Won't Chlorine Stay in My Pool? — diagnose ongoing FC loss
- Free Chlorine vs Total Chlorine — the CC reading you need for OCLT
- How Often to Test Pool Water — testing cadence during and after clearing
- Try PoolChem Tracker — calculate chlorine and chemical doses