Green Pool? Here's How to Clear It in 24 Hours
A green pool means algae has taken over. Don't drain it — follow these 5 steps to kill algae and get back to crystal-clear water in less than a week.
Why Is My Pool Green?
One cause: algae. A light green tint means it just started. Deep, swampy green means a full-blown bloom. Both are fixable without draining your pool.
Algae takes hold when chlorine levels drop too low — even briefly. Poor circulation, rain, heavy bather load, or skipping a week of testing can all trigger it.
Scan your test strip in Pooli to confirm your current chlorine and pH levels before you start. Low readings alongside green water confirms algae — and tells you how aggressively to treat it.
How to Clear a Green Pool: 5 Steps
Step 1: Brush Everything
Attach a stiff pool brush to your telescopic pole and scrub walls, floor, steps, and every surface you can reach. Algae grips surfaces tightly — you're knocking it loose and getting it into suspension in the water where chlorine can kill it.
Concrete pools: use a wire or algae brush. Vinyl and fiberglass: use a nylon bristle brush to avoid scratching.
Check off the Brush Pool task in Pooli after you're done.
Step 2: Test and Balance pH and Alkalinity
Before you shock, your chemistry needs to be in range or the shock won't be fully effective.
- pH: 7.4–7.6 (a bit lower is okay — shocking will nudge it up slightly)
- Total Alkalinity: 100–150 ppm
If either reading is off, adjust alkalinity first, retest, then adjust pH if needed.
Run your numbers through the Pooli Water Report to get exact dosing for your pool volume. Log each chemical you add.
Step 3: Shock the Pool (At Night, With High-Chlorine Shock)
This is the main event. You need calcium hypochlorite shock (cal-hypo) with at least 70% available chlorine. Standard granular shock won't cut it here — even if you normally use non-chlorine shock.
Dosing based on algae severity:
| Water Color | Shock Dose (per 10,000 gallons) |
|---|---|
| Light green | 2 lbs (double shock) |
| Green | 3 lbs (triple shock) |
| Dark green / swamp | 4 lbs (quadruple shock) |
Shock at night — sunlight destroys chlorine rapidly. Walk slowly around the pool perimeter and pour the shock in gradually. Or dissolve it in a bucket of pool water first, then pour.
⚠️ Always wear gloves and goggles when handling shock. Never mix different shock products in the same bucket.
After shocking, log the treatment in Pooli and check off Shock Pool.
Step 4: Run Your Filter Continuously
After shocking, run your filtration system for at least 8 hours overnight — ideally 24 hours straight until the pool clears.
Backwash your filter when the pressure gauge reads 8–10 psi above normal. For a pool this dirty, you may need to backwash multiple times.
Check back in the morning:
- Still green? Shock again with the same dose the following night.
- Cloudy blue? Algae is dead. Keep filtering.
- Clear? You're almost done.
If it's cloudy blue and you want to speed up clearing, add a pool clarifier and continue running the filter.
Step 5: Retest and Balance
Once your pool is no longer green, run a full water test:
- pH
- Total Alkalinity
- Calcium Hardness
- Free Chlorine
Scan your test strip in Pooli's Test Strip Scanning tool to get your readings, then follow the Water Report to bring everything back into balance.
How to Keep It From Happening Again
A green pool is almost always a chlorine maintenance failure. Prevent it:
- Test weekly at minimum — scan your strip in Pooli and it logs the trend automatically
- Add chlorine immediately when readings are low — don't wait until the next test day
- Don't skip weeks — algae can bloom in 24–48 hours of low sanitizer, especially in hot weather
- Run your pump long enough — aim for at least 8 hours per day in summer (use the Cost to Operate Calculator in Pooli to find the right runtime for your pump size)
Common Questions
Do I need phosphate remover? No. Phosphates are a food source for algae, but they're impossible to eliminate from pool water. The correct approach is killing the algae with chlorine — not trying to starve it.
Will baking soda clear a green pool? No. Baking soda raises alkalinity. Only chlorine kills algae.
Can I over-shock a green pool? No. More chlorine = better kill rate for algae. Follow the dosing chart above and shock as many times as needed on consecutive nights until the water turns blue.
How long does it take? With double or triple shock at night plus continuous filtration, most green pools clear in 3–5 days. A pool flocculant can speed this up — it sinks dead algae to the floor so you can vacuum it out directly.