A green pool is almost always an algae bloom. It happens when chlorine drops too low for a day or two, especially in hot weather. The good news: algae is easy to kill if you do it properly. The most common mistake is adding one bag of shock, waiting, and adding another a few days later. That never keeps chlorine high enough for long enough, and the algae comes back.
Step 1: Make sure it's safe to work on
If you can't see the bottom of the pool, keep everyone out, including pets. Make sure the pump and filter work and the skimmer and pump baskets are clean. Remove leaves and debris with a net: organic debris uses up chlorine you need for the algae.
Step 2: Test your water
You need three numbers before you start:
- Stabilizer (CYA): decides how high your chlorine must go.
- pH: bring it to about 7.2–7.4 first. Chlorine works much better at a lower pH.
- Free chlorine: your starting point.
If CYA is very high (over 80–100 ppm), consider replacing part of the water first. Otherwise you would need enormous amounts of chlorine.
Step 3: Raise chlorine to shock level, and keep it there
With stabilizer in the water, the shock level that reliably kills algae is about 40% of your CYA, with a minimum of 10 ppm:
| CYA | Shock level (free chlorine) |
|---|---|
| 0–25 ppm | 10 ppm |
| 30 ppm | 12 ppm |
| 40 ppm | 16 ppm |
| 50 ppm | 20 ppm |
| 60 ppm | 24 ppm |
| 80 ppm | 32 ppm |
Use the shock calculator (choose "Green / algae") to find the exact amount for your volume. Liquid chlorine is the best choice here, because cal-hypo adds calcium and dichlor adds even more stabilizer with every dose.
The key: test free chlorine two or three times a day (morning, afternoon and before bed) and add more every time it drops below the shock level. Algae consumes huge amounts of chlorine at first, so you may add several full doses on day one.
Step 4: Brush and filter, every day
- Run the pump 24 hours a day until the pool is clear.
- Brush the walls, floor, steps and corners at least once a day. Algae hides in places with poor circulation.
- Clean or backwash the filter whenever its pressure rises 8–10 psi above normal. A dead algae bloom clogs filters quickly.
- The water typically goes from green to blue-gray to cloudy white before it clears. That is normal: it means the algae is dead and the filter is removing it.
Step 5: Know when you're done
Stopping too early is why algae comes back. Keep chlorine at shock level until all three are true:
- The water is clear: you can see the main drain clearly.
- Combined chlorine is 0.5 ppm or less.
- You lose less than 1 ppm of free chlorine overnight (test at dusk and again in the morning, before the sun is up).
When you pass all three, stop adding chlorine and let it fall back to your normal level. Check your normal target with the chlorine calculator.
How long does it take?
A light green pool often clears in 2–4 days. A dark green or swamp-like pool can take a week or more, depending on how well your filter works. Flocculants and clarifiers can speed up the last, cloudy phase, but they do not replace chlorine: dead algae is the goal, and only sustained chlorine gets you there.
How to stop it from happening again
- Keep free chlorine at or above the minimum for your CYA every day, not just after it rains.
- Test more often in hot weather and after heavy use.
- Watch your CYA: if it keeps rising from trichlor tablets, switch to liquid chlorine or a salt system.
- Run the pump long enough each day and brush once a week.
Safety first: Always read the product label. Add chemicals to water, never water to chemicals. Add one chemical at a time with the pump running, never mix products (especially chlorine and acid), and keep everyone out of the water until levels are back in range. Wear gloves and eye protection when handling acid or shock.
Sources and methodology
Last reviewed: November 2026. Doses are calculated from the chemistry of each product (molecular weights and available chlorine) and cross-checked against manufacturer and industry dosing charts. Recommended ranges follow CDC guidance for home pools and hot tubs.